I had 4 females living together once and one day 3 of them mauled the smallest one. I got her her own cage but they did manage to rip her ears off. They were in a 20 gallon long tank.
It could be something is wrong with your setup. Or, like Mousey said, you could have a pregnant mouse or a secret male.
But sometimes you can come acrossed just a plain mean mouse.
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1 Rat: (Ace)
1 RC Hamster: (Hannibal)
3 Cats: (Tao, Drew, Jak-ke)
2 Ferrets: (Fisher, Georgie)
1 Betta: (Maxwell) + (5 Platies and 1 Molly)
Rest in peace all my little Mice: Tempo, Aaron, Tatiana, Pei, Shi, Orange, Spotty, Lee Hyun, Key, Calvin, Chun, Jiro, Hero...
And my past Dookers: Lulu (my heart), Frodo (my old man), MoneyPennies (the first one), ect...
Aswell as all my lost Cats (my beloved Haru, ect..) and Bettas (Stahl the creepy one, ect..)