Have you ever played "Mortal Kombat" at a house party? As super-violent as it is, people generally are pretty chill about winning and losing... as long as its person-to-person.
On the other hand, I've played a game of Chess online once, and after making one or two good moves, I've been called homophobic names before the guy rage-quit on me.
You can take the least stressful activities, and if they're online, you WILL find people screaming death-threats at each other for some insane reason.
I think the online environment makes people somehow susceptible to stress and anger. At least, I personally go online to relax after the day... but with that relaxation... my inhibitions are released as well. I'm an introvert by nature, when I'm in public, I'm keeping tabs on my own behavior, watching what I say and trying to not be an asshole.
But when I'm at home and just browsing the internet, I'm not going to be holding back my words or thinking about other people's feelings. I'm completely tired of doing that all day. Plus, I'm at home, and (mostly) anonymous online. Why should I care about how I look or about other people's feelings?
I know it is lazy, but I'm just trying to be honest. And if anyone disagrees, please tell me about your experience with "Online Chess" vs "In-Person games of Mortal Kombat". I can almost guarantee you, the behavior of online-chess players is probably worse.
Besides, the whole Youtube experiment has proven that "anonymity" means absolutely nothing to your typical troll. Forcing people to display their names has only made the system worse.