I think landing on a back is one of the least likely places to land. It has never happened to me.

On the head is much more common from a bicycle accident. If you are going at a decent speed and the bike is stopped suddenly, your legs get brushed up as you go over the handlebars. That starts your body rotating, and landing on the head becomes very likely.

That one did happen to me. Details here, helmets. It wasn't pretty.