I have been riding over 40+ years. Had, had my horse for 7 years, was out riding alone (wich I always have done not a big deal to me even when I was a kid I rode alone). For some reason my horse became a bucking bronco, resulting in my back being broken in three places my hip in two places emergency surgery and hospital stay.
It was after my accident that my horse taught me that no matter how much we brag on our horses being those ben there done that, will do it all over again, or bomb proof horses. A horse is a horse they will spook, buck, rear, kick bolt at any given time no matter what training they have or how much trust you have in each other. I actually got back on my horse six months after the accident. I didn't ride him much because of my injuries.
My point is don't beat yourself up or wonder or try to find a reason for what happened. It only makes you worry more. After six years I still don't know why my horse did what he did. I came to the conclusion that he is just a horse and horses will do those things.