Why 3 of my rats ate the other one?

Okay this happened a while ago, but someone reminded me about it and I wanted to know.

I had 1 rat that I bought from a pet store and I had her for about 3 months where she lived alone. Well my younger brother and sister kept begging my mom to get them a rat to and she gave in. She adopted 3 litter mates from the shelter and put them in with my rat. (the cage was really big, so they each had their own little house and toys ect.)

Well for the first month everything was good, they shared food, water, and treats with each other. Never scratching or biting at one another. After the first month my brother and sister got tired of them (like i mentioned to young to have rats) and I had to care for all of them. Which was fine since i figured it was good for my rat (angel) However, i didnt handle the others as much as angel because the others didnt like being held for very long and were finiky (nor would they just sit on my shoulder) but I tried to handle them all each day so they would stay sociable.

Well, one day I go to feed them treats and angel had been attacked, (the other rats had chewed her head) and it was horrible.

Why would they do this? (I know it was wrong but I made my mom send them back to the shelter, angel was really special to me and the others just made me mad and sick at the same time)

The only differences I could see is that Angel was bigger than them (she was a dumbo and they were just standard)
Angel was a bit older (but only by a few months)
And she came from a store and they came from the same litter.

I dont understand why they would get anlong for a month and then just turn on her like that.
It happened about 7ish years ago.