People just always feel the need to label others with names because of their own insecurities. Someone may call someone a "knife nerd" or "computer nerd" or "gun nut" because the person has an above average amount of knowledge on the subject and it's probably one of their main hobbies. In reality many people who use these terms are merely intimidated by people with extensive knowledge on subjects because they themselves are extremely bland and boring, lacking any real fulfilling knowledge in some sort of extensive way.
I consider myself extremely knowledgable with certain things, firearms, computers, cars, and knives. I don't know nearly as much about all things knives compared to you guys, but to the average person, a whole lot. Anyways, people who have extensive knowledge on subjects like these would rarely call someone else a "xxx nerd" unless it was in a joking sense. We most likely would call someone an "expert" or say that they're just very knowledgable about something. Many people fear this and it makes them feel inferior when they realize how much more someone else knows about certain things compared to them. Maybe it's their subconscious, I don't know.
But I do know people are intimidated by others that seem superior to them in certain ways, and intelligence is one of those things. You'd be surprised how many people just don't know jack about much of anything except their 9-5 and they're bored as hell because of it. It's an easy way for someone to make their self feel better by implying that an expert of a certain hobby is lower then them, ie a "nerd" or a "nut". Respectable people will use terms like "aficionado" etc.
