A well socialized dog will not mind having their butt sniffed, it's how dogs greet and how they find out information about each other. I have met a few dogs who are afraid of other dogs sniffing their back ends. They probably had a bad experience at some point, possibly being bit by another dog on the rear end. They snarl and air snap at any dog that gets close to their butt. When my dog encounters these dogs he just rolls onto his back and that usually makes the other dog calm down.