I have a 8 year old gelding, recently bought him two weeks ago. He is a 2D barrel horse & I've watched him countless times being rode. Saturday, I tacked him up for my boyfriend to ride. I checked for any soreness, like I always do. I've ridden him twice also with no problems. He stood for tacking, & mounting, but as soon as my boyfriend's butt hit the seat of the saddle he blew up. Bucking, spinning, rearing. After about 5 or 6 bucks my boyfriend lost his seat & fell off the side. The gelding then went through a fence, galloped the field a few times, then stood in the middle, calm as can be & let me walk right to him to be caught. I'm not sure if he was having a bad day, or if he just didn't like my boyfriend, or what. My friends & I did leave before him, but he could still see the other horses.. I'm not sure if he is that horse sour to blow a fuse or what?