The root of all aggression is fear 90% of the time. So (even if there is not one incident to pinpoint), somewhere along the line your dog has had some negative experiences with strangers. Sometimes it is a series of physical abuse by strangers that have made them not trust human’s intentions. But for many it is as simple as them being a shy insecure dog, and time after time people they did not know were constantly invading their space by petting them and or picking them up. Meanwhile, the dog was giving all the proper signals of being nervous and requesting some space. However, these clear canine signs were being ignored or not recognized by the humans. This results in the dog having to resort to aggression, which when he/she does….typically humans back up quick, so this style of communication works much better for the dog.