CL's on target.
While certain dog breeds are more powerful, protective, and even loyal than other breeds, that does not mean that those breeds (pit bull terriers, Rottweilers, Doberman pinschers, and even German shepherds, among others) are automatically hard-wired to be aggressive.
Those breeds may be predisposed to aggression, but no dog comes to the handler trained out of the litter.
Those pit bulls had to have been trained in some regard, or, perhaps better said, the lack of training and socialization could very well have been the reason for the dogs' attack. Further, having that many dogs in one place is an invitation to disaster.
Dog packs, as an entity, have a dynamic all their own. These dynamics and tendencies can't be predicted with 100% accuracy.
Therefore, it's incumbent upon the human handler and responsible dog owner to take appropriate measures to protect and ensure the safety of everybody around those dogs. You simply can't expect a 12-year-old child to control a dog pack. Ain't gonna happen.
The irresponsible adult dog handler/owner in this case is the reason for this tragedy. This individual needs to be held accountable for manslaughter.