Why do people call other people nerds?

Though one may blame it on glasses or a respectably uninhibited display of intelligence, people call others nerds when said "nerds" act in a way that undermines social norms. Nerdy behaviour is usually characterized by obsessive behavior, inability to pick up on social cues, and lack of common sense or 'street smarts.' Generally a nerd is booksmart but socially inept.

Having to ask that question puts one in the 'nerd' category.