I... have no idea. I had a Standardbred a few years ago that would buck going into canter. Turned out it was the saddle, which a fitter had told us would be fine, but my god could he buck! Not a ride went by that I wasn't ditched for such a long time, and then finally I started figuring out how to stay on and the bucks got bigger and bigger. And then I rode a pony for about 18 months that would buck as an evasion, to get out of work because he was such a lazy little sucker. He would buck, I would whack him hard with the crop, he would buck again, I would whack him... lol funny pony. He got me off more times than I can count. He also had this real dirty little habit of EVERY SINGLE TIME I tried to gallop him, dropping a shoulder, shying violently, and THEN bucking and taking off in the opposite direction that got me off every time. And then Monty's bucked me off once.
I've been dropped countless times by horses for reasons other than bucking, but have never once been thrown by a spin or a rear (or both combined), which I'm pretty proud of. I've also, to my memory, never actually had a fall that did not involve a horse doing something wrong.... but I've been riding since a pretty young age and never really considered falls any kind of significant so I probably have forgotten a great many.