Yeah the guy at the back should not have been close enough to rear end her. But give him a break, If you are going to stop dead, not slow down for a technical section but stop dead, do it at the side of the trail. If you stop in the middle of the trail you should expect a problem or be causing one I reckon.
Anger management training is required and I reckon the guy at the back can give it. I would have lost it well before he moved on.