When one dog meets another dog on street, there is often some etiquette or lack there of which ensues. In one instance a dog may bare its teeth, walk a few steps, and finally bump the other dog with its butt. When the other dog fails to respond, it bumps the other dog again and again. The other dog was not hurt, and decides to back off a bit.
So, a dog fight has been avoided. Goes to show a little bit of tolerance, and butt bumping can go a long way.