What's in a name?

Was just curious how someone comes up with a nickname like Butch or Bud. Mike is naturally short for Michael, Jim is short for James, but what is Bud or Butch short for? It's one thing to say "Hey Bud" to someone, but another thing to be like, "Hey this is my friend Bud." You don't hear those nicknames much anymore though. But why are Butches and Buds reserved for janitors and auto mechanics? Maybe that's why nobody uses those nicknames anymore.