##05AB1E, 20 13 bytes##
A.pvy¦«=}gF, Explanation
A.pv } # for each prefix of the alphabet y # bifurcate (push it and it's reverse) ¦« # remove first char of the reversed string and concatenate = # print while leaving on the stack g # length of top item (the longest string) F, # pop and print that many times (prints the lines of decreasing length) Saved 3 bytes thanks to Adnan.