Boobs that big can be fairly heavy (7-12 lbs), plus if she was sitting on his chest it is possible that she chould have trapped him under them + some of her body weight. 70-90 lbs suddenly dropped on your face during / after a sexual encounter could give the attacker the upper hand.
This is a lesson for all men, BOOBS ARE WEAPONS OF MASS DESTRUCTION!!! and should be monited by the UN.
I am waiting for the fox news story about the dude who stabbed 3 people to death with his penis.