A Shorter Answer:

You probably either have a long running transaction running (Index maintenance? Big batch delete or update?) or you are in the "default" (more below on what is meant by default) recovery mode of Full and have not taken a log backup (or aren't taking them frequently enough).

There could be other reasons but these are the most common. This answer begins to dive into the most common two reasons and gives you some background information on the why and how behind the reasons as well as explores some other reasons.

A Longer Answer: What Scenarios can Cause the Log to Keep Growing?

There are many reasons, but typically these reasons revolve around following patterns:

Not Understanding Recovery Models

(Being in Full Recovery Mode and Not Taking Log Backups)

While this answer is not a deep dive in SQL Server recovery models, the topic of recovery models is critical to this problem. In SQL Server, there are three recovery models - Full, Bulk-Logged and Simple. We'll ignore Bulk-Logged for now but for the purposes of this question, we can sort of say it is a hybrid model and most people who are in this model are there for a reason and understand recovery models. The two we care about and their confusion are the cause of the majority of the cases of people having this issue.

Before we talk about Recovery Models - Let's talk about recovery in general. If you want to go even deeper with this topic, just read Paul Randal's blog and as many posts on it as you want. For our purposes - a few points:

1. The log file is there for recovery. For the rolling forward and rolling back of work that was either done before a crash or restart and the work that was started but not finished after a crash or restart. It is the job of the transaction log to see that a transaction started but never finished (rolled back or crash/restart happened before resolution of transaction). In that situation It is the log's job to say "hey.. this never really finished, let's roll it back" during recovery. It is also the log's job to see that you did finish something and that your client application was told it was finished (even if it hadn't yet hardened to your data file) and say "Hey.. this really happened, let's roll it forward, let's make it like it was" after a restart. Now there is more but that is the main purpose.
2. The other purpose for a transaction log file is to be able to give us the ability to recover to a point in time. If this transaction log contains the records of transactions that have been started and finished for recovery, SQL Server can and does then use this information to get a database to where it was before an issue happened. But that isn't always an available option for us. For that to work we have to have our database in the right recovery model, and we have to take log backups.

Onto the recovery models:

Simple Recovery Model - So with the above introduction, it is easiest to talk about Simple Recovery model first. In this model, you are telling SQL Server - I am fine with you using your transaction log file for crash and restart recovery (You really have no choice there.. Look up ACID properties and that should make sense quickly), but once you no longer need it for that crash/restart recovery purpose, go ahead and reuse the log file.

SQL Server listens to this request in Simple Recovery and it only keeps the information it needs to do crash/restart recovery. Once SQL Server is sure it can recover because data is hardened to the data file (more or less), the data that has been hardened is no longer necessary in the log and is mared for truncation - which means it gets re-used.

Full Recovery Model - With Full Recovery, you are telling SQL Server that you want to be able to recover to a specific point in time, as long as your log file is available or to a specific point in time that is covered by a log backup. In this case when SQL Server reaches the point where it would be safe to truncate the log file in Simple Recovery Model, it will not do that. Instead It lets the log file continue to grow and will allow it to keep growing, until you take a log backup (or run out of space on your log file drive) under normal circumstances.

There are rules and exceptions here: We'll talk about long running transactions below, but one to keep in mind for in full mode - If you just switch into Full Recovery mode, but never take an initial Full Backup, SQL Server will not honor your request to be in Full Recovery model. Your transaction log will continue to operate as it has in simple until you switch to Full Recovery Model AND Take your first Full Backup.

So, that's the most common reason for uncontrolled log growth: Being in Full Recovery mode without having any log backups. This happens all the time to people.

(Why does it happen all the time? Because each new database gets its initial recovery model setting by looking at the model database. Model's initial recovery model setting is always Full Recovery Model - until and unless someone changes that. So you could say the "default Recovery Model" is Full. Many people are not aware of this and have their databases running in Full Recovery Model with no log backups, and therefore a transaction log file much larger than necessary. This is why it is important to change defaults when they don't work for your organization and its needs)

You can also get yourself in trouble here by not taking log backups frequently enough. - Taking a log backup a day may sound fine, it makes a restore require less restore commands, but keeping in mind the discussion above, that log file will continue to grow and grow until you take log backups. You need to consider your log backup frequency with two things in mind:

1. Recovery Needs - This should hopefully be first. In the event that the drive housing your transaction log goes bad or you get serious corruption that affects your log backup, how much data can be lost? If that number is no more than 10-15 minutes, then you need to be taking the log backup every 10-15 minute, end of discussion.
2. Log Growth - If your organization is fine to lose more data because of the ability to easily recreate that day you may be fine to have a log backup much less frequently than 15 minutes. Maybe your organization is fine with every 4 hours. But you have to look at how many transactions you generate in 4 hours. Will allowing the log to keep growing in those four hours make too large of a log file? Will that mean your log backups take too long?

Long Running Transactions

("My recovery model is fine! The log is still growing!)

This can also be a cause of uncontrolled and unrestrained log growth. No matter the recovery model, but it often comes up as "But I'm in Simple Recovery Model - why is my log still growing?!"

The reason here is simple, if SQL is using this transaction log for recovery purposes as I described above, then it has to see back to the start of a transaction. If you have a transaction that takes a long time or does a lot of changes, the log cannot truncate on checkpoint for any of the changes that are still in open transactions or that have started since that transaction started.

This means that a big delete, deleting millions of rows in one delete statement is one transaction and the log cannot do any truncating until that whole delete is done. In Full Recovery Model, this delete is logged and that could be a lot of log records. Same thing with Index optimization work during maintenance windows. It also means that poor transaction management and not watching for and closing open transactions can really hurt you and your log file.

You can save yourself here by:

• Properly sizing your log file to account for the worst case scenario - like your maintenance or known large operations. And when you grow your log file you should look to this guidance (and the two links she sends you to) by Kimberly Tripp. Right sizing is super critical here.
• Watching your usage of transactions. Don't start a transaction in your application server and start having long conversations with SQL Server and risk leaving one open too long.
• Watch your implied transactions in your DML statements. UPDATE TableName Set Col1 = 'New Value' is a transaction. I didn't put a BEGIN TRAN there and I don't have to, it is one transaction that just automatically commits when done. So if doing operations on large numbers of rows, consider batching those operations up into more manageable chunks and giving the log time to recover. Or consider the right size to deal with that, or perhaps look into changing recovery models during a bulk load window.

Getting Info on Your Cause

There are other reasons as well and I'll add those to this in a coming edit or perhaps someone else will answer with more details on one or edit this themselves. But they involve other technologies such as mirroring, log shipping, replication and other reasons.

That said - there is a way you can analyze your reason for log growth reasons and see what they are - By Querying the [sys.databases][5] catalog view you can see information pertaining to the reason your log may be waiting to be truncated and re-used (the opposite of growing forever). There is a column called log_reuse_wait and this column can actually be pretty useful. It is a TINYINT column and a log_reuse_wait_desc column with a description of the wait reason. It is my hope that most of these reasons get discussed further in this Question but the full list of reason code to reason description mappings (from the linked Books Online article in SQL Server 2012) with a few of my notes:

• 0 = Nothing - What it sounds like.. Shouldn't be waiting
• 1 = Checkpoint - Waiting for a checkpoint to occur. This should happen and you should be fine - but there are some cases to look for here for later answers or edits.
• 2 = Log backup - You are waiting for a log backup to occur. Either you have them scheduled and it will happen soon, or you have the first problem described here and you now know how to fix it
• 3 = Active backup or restore - A backup or restore operation is running on the database
• 4 = Active transaction - * There is an active transaction that needs to complete (either way - ROLLBACK or COMMIT) before the log can be backed up. This is the second reason described in this answer.
• 5 = Database mirroring Either a mirror is getting behind or under some latency in a high performance mirroring situation or mirroring is paused for some reason
• 6 = Replication - There can be issues with replication that would cause this - like a log reader agent not running, a database thinking it is marked for replication that no longer is and various other reasons. You can also see this reason and it is perfectly normal because you are looking at just the right time, just as transactions are being consumed by the log reader
• 7 = Database snapshot creation You are creating a database snapshot, you'll see this if you look at just the right moment as a snapshot is being created
• 8 = Log Scan I have yet to encounter an issue with this running along forever. If you look long enough and frequently enough you can see this happen, but it shouldn't be a cause of excessive transaction log growth, that I've seen.
• 9 = An AlwaysOn Availability Groups secondary replica is applying transaction log records of this database to a corresponding secondary database. About the clearest description yet..