Chances are that it's pain related. I would check for stone brusing first, I've known this to make horses buck
Sometimes even a tiny little "ouchie" could make a horse buck. Tana had a small, and I mean small cut on her front leg and she bucked for the only time when I rode her for 15 mins with that on her leg. It has hardly a scratch.
But it could also be that your boy was just having a bad day, that does happen. Maybe try getting on him again and see what he does!
Hope that helps!