If you ask me, it comes from the fact that software geeks are primarily young men, many of whom were emotionally abused by their peers while growing up. When you think the world only values your intelligence, you go out of your way to prove that you're smarter than everyone else.
I'll answer the, perhaps rhetorical question, by defining a new law: "Any online community that doesn't actively encourage its users to be nice, and civil, to each other is bound to become a cesspool.".
It can be a very serious problem when this happens within a team, IMO.