I can't believe people are still playing Counter Strike. We had servers for the original version (Half Life 1 based) way back in the dorms in like 2000/2001.
In short, I think it comes down to the fact that people do not have to maintain a social persona when going through the match making queue. I think people are default assholes and don't behave nicely unless they know they'll see you again/you become familiar.
A lot of the communities I've seen start off super friendly and fun when they're small (because you're playing the same small pool of people) and then reach a tipping point when toxicity takes over because you rarely bump into the same people over and over again.
I think that could be solved by modifying match making to preferntially rematch you with the same people over and over again, in effect simulating a smaller world.
Separately, toxicity is contagious, which also agrees with my observation that larger communities are more toxic. I wonder if Valve has done any analysis on how behaviour is affected after interacting with tilted/toxic players.