I think it's a dominance thing. That's how packs of dogs (wolves...) bring down prey in the wild. They attack the sensitive, vulnerable parts. A dog will get the deer/elk.. by the nose/mouth area, another one the hindquarters and one of them goes for the jugular. You can see that a lot if you watch the animal planet or national geographic or other wildlife shows.
I think for a dog, it's just a natural instinct to win a fight - play or not.
saje