Why does my dog growl when I pet his back legs?

There may be a lot of other reasons why your dog growls at people who touch you but here are the one's I can think of right now

Your dog sees you as part of his pack, and no one else. In other Words, he sees you as the alpha and more then likely sees himself as beta. Everybody else is an outsider, and when one of them touches you,s/ he may think they are threatening you, and he goes to 'protect his alpha'

Your dog sees you as a puppy... maybe even his/hers puppy. I guess you can see why he/she would growl in this case.

Your dog may veiw YOU as Beta and HIM/HER as alpha and may be thinking that the person is trying to steal you into their pack.