I think it was the broody mama who pecked her or at least started it. She was done being mama and didn't want them bothering her and she is in general a nasty chicken

Apparently I didn't check the little chick over enough at first and only saw the head "scalping". I didn't see the chest wound and the back of the neck. About 24 hours after, we discovered that she had flystrike and had to pick out all the little worms. That was extremely difficult for my daughter to experience, but she wanted to help the chick. I think that is what really killed the chick. She died right after we got all the larva off her. Poor little dear. But we tried to console ourselves by remembering that she had a wonderful little life playing outside with her little chick sisters.