I am so sorry to hear about that and your mother's neighbor has my sympathies.
I own a pit bull/boxer mix and she is a sweetheart. I wouldn't trade her for the world. I agree with
CritterWhisperer it would be hard to tell what happened without all of the details. In my opinion it could have a lot to do with the age of the dogs.
I've had my dog since she was six weeks old and she's seven years old now. She used to love to go to the vet but as she got older she got to the point that she does not like it. It has gotten so bad that they have to sedate her to tend to her. The only time she has ever tried to bite someone was at the vets office when the vet pulled her leg back a little to far and hurt her. If they are older dogs they could have been hurting and he touched where they were having pain. I know the bully bred has hip problems when they get older which is one of the things we are watching out for.
As
CritterWhisperer said though it could be many other reasons as well. There are just too many things to tell exactly why it happened.