Bad warband prop collisions, there is no real clear way to fix this issue besides removing a bunch of props through trial and error which I'm not inclined to do. It could be anything to the wall or that barrel, or even the ballista to the left. When you resize objects the chance of something strange like this increases as in this case the DB are hitting the collision box for one of the props nearby though none are apparently visible.
Short answer is I don't know why this occurs here and not on the other side.
Short answer is I don't know why this occurs here and not on the other side.