I'm not sure if the damage you do to the person makes any difference. I just think a person is a person, and doesn't matter how much armor or HP they have for determining if you get reared or not when bumping them.
The only thing I've noticed that matters is how fast you're going and how much charge the horse you're on does. For courser it can be risky trying to run over more than one person within 2 seconds or so time.