I have to wonder if the child's mom was menstruating and that was the catalyst that switched the family pet into a monster?
...that and the fact the father/pack-leader was not home at the time.
The story doesn't say how long they had the dog, its sex or the other dog's sex.
There were two pit bulls in the house and I heard on the radio that the six month old female pit bull was in heat. I have not found anything about this written.
Any and all such mundane circumstances being possible triggers are just more reason why ‘pitties’ should not be in residential households.