It could be a territory thing. Are you putting them in directly or are you putting them into a freshly cleaned cage? I've noticed with my mice that if I put the mice in without cleaning the cage, there will be extremely nasty fights. However, if I clean the cage, put the new mouse/mice in, put food in, let them wander around a bit, then add the old mice, there's much less fighting. There's a few minor spats, but nothing serious.