What kinds of things go wrong on the back nine? (Long game, short game, everything?)
Could be any of the things that have already been mentioned.
Some of the seniors where I play obviously start getting tired on the back nine.
For the younger guys it's just about the opposite. They start to get loosened up, their swing gets much longer, and they start trying to hit the ball harder. In the early part of the round they are much more under control.
My typical round is usually good shots and fairly conservative play (for me) for the first few holes, start to blow up a little in the middle when I get a little too full of myself (and start getting greedy), and get back under control the last few holes (when I figure out I'm not good enough to be greedy).