I gave a horse a clover and went to hop on, i noticed it was pathing around a cliff trying to get to the other side just naturally. I went to "giddy up" and immediately got thrown off with damage. I was then stuck inside the horse hitbox and couldn't move, had to hearth out. seemed like the cliff had something to do with it.
Not sure if this is actual bug or just really bad luck, I heard horses can kick you off before stam runs out but i've mounted like 80 times and been fine.