I've been told he does it from fear, but I don't know because he never seems nervous.
We have tried all sorts of jumps, crossrails, single verticles, double verticles, panels.. he does it for everything. I have seen him jump up to 2'6.. he still overjumped but not as much. It's a pretty frustrating problem, but I'm learning to cope better. I can sit it so it's not particulary dangerous, and he's really good jumping.. doesn't go too fast and is perfect steering.