Sometimes I think it's more how the “victim” acts than anything else. Some people seem to exude fear of dogs that triggers the dog. Running from a dog triggers attacks as well. I've very rarely had a dog not back down when I commanded them to, even “mean” police dogs. Little yappy dogs though are sometimes just insane.
On the other hand, my "beautiful" Labradoodle is the sweetest dog I've ever had, and I've had many through the years.