It's just a joke, because random newbie posters would show up and get all offended by the snarkiness of the board, especially from those posters. Aurora and Anita especially don't put up with just plain stupidity, like someone asking the same question that had just been asked two posts down. Their responses resulted in them being called "mean girls" by those posters, which those four embraced.
Edit: Forgot to give the actual why behind it. People started picking "labor buddies" and they decided to be the "mean girls" group. Funny enough, it's not even September yet and 3/4 have already had their babies! Eek.
I think first and foremost it's because they are labor buddies. The actual mean girls picture, I think, stems from the fact that those girls are pretty "tell it like it is" which some others have determined to be "mean?"