The group of mean girls usually do this to feel powerful, as a form of sport or simply because they can. It is cowardly, bullying and sociopathic behavior since it is usually a GROUP of powerful girls against ONE powerless girl.
Rarely, one mean girl can attack another girl but in this case the mean girl is usually MUCH MORE POWERFUL than her victim and attacks her victim specifically because she knows the victim is powerless and unable to defend herself.
Originated from the movie "Mean Girls".
Mean girl 2: I know what will make you feel better. Let's go mean-girl that girl over there. She's all alone and she looks different from everyone else. She stands out and will make a perfect victim.
Mean girl 1: Okay! But let's get someone else to join us first so it will be 3 against 1. That way we will be sure to win and mean-girling some poor sob will make me feel GREAT about myself. Yay!!