LOL. Sorry, but that's the only thing I could think of. Basically you're moving faster so you have quite a bit more momentum when you come off.
When I came off at a gallop I was actually thrown. My darling horse was trained to buck on command, well he also would do it occaisionally in "celebration". We were racing my sis, won, and he bucked. I don't remember anything except waking up on the ground, but apparently I flew quite a ways through the air before hitting the ground. I landed on my back and my head snapped back into the ground. My sis thought I had been killed, but I came to shortly afterward and was "fine." *Note, this is not Soda who did this.