Was he on the horse? It's pretty easy to flip a horse, especially a young, unbalanced horse. If the horse was trying to go to her friends and the guy reined her in with a long leverage bit, AND kicked her to go forward, she had no where to go but up. I've seen this happen twice at horse expos! Once she finds that's an escape from pain, she'll be more likely to try it again in the same circumstances, but rearing is a lot of work, and I bet if she learned a clear go forward cue during ground work, she's get over it.
If he's tried to move her forward without holding her head, he might have avoided it. Of course saddle fit and idiots training her could be compounding the problem! Cherie
