Why would my rats eat one of thier cage mates?

okay, so i had 3 femalse and 1 male rat all in the same HUGE cage for quite sometime, they have been cagemates since ive gotten them. then today i found one of the females on the top level of the cage. the others had eaten her inside and out, just the skin and a few what looked like spine and leg bones left. i dont understand why they all of a sudden would have done this, being cagemates so long..however when i tried to take the body out one of the back legs was stuck in the cage. so maybe she was stuck and injured and they killed her off? or maybe she was pregnant?? or maybe one of the other females is pregnant?? please help...i need my question answered...