See Seattle Times story here
My girlfriend works with the widow, and from her description it sounds like the group coming the other way was drafting, and the second position rider got a bit out of control as they were trying to slow. That's a guess, though; she wasn't really able to remember what happened too clearly. For whatever reason, he never got his hands down and so went face-first into the ground. Apparently most of the bones in his face were broken, resulting in massive brain trauma, so his helmet didn't matter. I guess the moral is: auto-free paths are no guarantee of safety.