Well, it was a dirt bike, so it was probably a dirt bike helmet with no shield. I guess if at the right angle, it could have caught through the opening and might have been able to snag him at around the upper lip area. Which, if you look at the large opening of a dirt bike helmet and the way the front piece that covers the mouth seems to stick out further than a street going full face helmet, it really does seem possible for the rope to make its way through the opening and catch you between the nose and the upper lip area.
I was also wondering if the rope caught the underside of the helmet, pulled the helmet up over his head and the rope ended up catching him at his upper lip/nose.
I guess?