Edit: I figured out what was wrong, hehe. I had a black background at the back and he was seeing himself in it's reflection and getting mad. But now that I took it away, he has nothing to get mad at so he's not hiding back there anymore. Haha!
Good to hear that you found the cause--when normally bold fish start hiding it can be a sign that something is seriously wrong medically. Usually we don't figure out that our fish are sick until they are very sick, because it is instinctual for them to hide their condition because the strong will always be inclined to pick off the weak. I would watch him extra closely--glad you think he's allright, though.