A buck is when their hind legs lift their back, forcing the rider forward. This was caused by the horse rearing, which is going up with their front legs. This horse is actually not really misbehaving. She's yanking his head back by pulling on the bit really hard, so he's lifting it up. She continues to pull, he continues to lift just to escape the pain. He's just giving in to make the pain stop, when he accidentally lifts too far and flips. Flipping over like this can really injure a horse, so fuck this fat. I hope the horse broke it.