I had a pony who used to spook on purpose. Back when I was very young and just learning how to ride everytime he spooked I would get off and put him back because I was scared. He learned very quickly that he could get me off that way. To this day he will randomly shy and jump at things that make no sense. It all depends on the rider too, he will spook, buck, and rear with me riding. But I put a beginner on him and he is a bomb proof well behaved pony -_-.
Anyways, his spooking ended up breaking my leg when I was about 14. We were cantering through some mud he spooked and slipped an fell ontop of me. Looking back I have no idea if he did it on purpose or he actually spooked. Guess I will never really know.