Defensive writing is the straightforward consequence of increased exposure to assholes. While in real life assholes are marginalized, and mainstream discussions involve only reasonable people, Internet media enable assholes to masquerade as normal people (they hide behind an ordinary appearance, with little chance to know they will become irritating), increase their reach (they can pick on anyone, anywhere and at any time, rather than irritating a limited circle of acquaintances) and reduce social defenses against them (for instance, they can easily return with a pseudonym after being banned from a forum, which in itself is a much milder consequence for antisocial attitudes than, say, being beaten up or losing their job in real life).