Check her and others for mites and lice. Roost mites only come out at night so you need to check for them after dark. They normally like the vent area because of the moisture so check there first. Roost mites will run from the light of a flashlight.
Is it possible a possum is getting into your coop and chewing on that chicken’s vent area while they are on the roost at night? Possum’s normally attack the vent area but usually when they do they kill the chicken. Maybe it is a baby possum? I’ll also throw out the possibility it is a rat doing that, but I don’t know enough about what rats do to really know.
If a chicken is wounded and bleeding the others will often peck it and kill it. You saw that.
It’s possible that hen had a partial prolapse, they started pecking, and she then drew it back inside. By then she was wounded. With it happening to two of them, that sure doesn’t sound likely. I’d suspect something else is going on.
Good luck with it. Sometimes these things can be hard to figure out.
