I can’t really put my finger on it, because it wasn’t that the game was swarmed by evil people or anything weird like that. I am thinking perhaps it’s easier to mind small burps before they turn into catastrophic spew with a small number of players?
At the risk of overreading my experiences, I’m not sure it takes much more than a small handful of people who treat faction conflict as OOCly competitive rather than cooperative. Someone decides they’re going to get a definitive win over the enemy by leveraging their superior game stats to permanently eliminate someone on the other team, and now everyone’s thinking about that possibility.
People who are competitive by nature have something to point to to rationalize pushing their own stats further (and once you have that hammer, you start to look for nails). People who aren’t suddenly realize that their investment in the character is at risk from people with more system mastery, and become less willing to engage outside their comfort zone. And so on.