My quess would be that the round fell in JUST the right orientation on JUST the right size rock to strike the primer. Odds are you could take a case of ammo, drop them one by one and not have this happen again for a LL-OO-NN-GG time. But statistics caught up to you this time. PS. if you think about the physics involved; the bullet, having the highest mass, would have gained the least velocity from the detonation.