I was told by my Pediatrician that the largest amount of dog bites to children happen when the child is two to three because the dog no longer see the child as a baby but rather part of the pack.
The dog may well be fighting for it's place.
I think you are right. The only dogs that ever attacked me were a set of two, kept by a family. They revert to the "pack mentality" when there is more than one.
I have two new lab-akita puppies, and I intend to keep a close eye on them, no matter how gentle they appear. I can really see the "me-too" coming out, even at 9 weeks.