It's some weird anomaly that I haven't figured out either.
I've thought about it for a few years now off and on.
Something about l4d2 turns everyone into giant assholes, it's like the PERFECT formula to do it.
Even playing it with friends, it turns everyone against each other the very second a stray bullet hits a teammate, or the very second someone lags behind and has to kill something, or the very second someone gets downed at the worst possible time.
It sends everyone into a frenzy against the person who made the mistake, no matter how big or small.