I had this happen to one of my silkies still in the brooder. It was the other chicks that did it, as they were in the house and nothing else could have happened to them. It IS horrifying.
The only thing I can figure is it had a little food on its beak and you know how they peck each others beak sometimes? I guess it just got out of hand?
So you might be looking at the others as culprits, once they have started this bizarre behavior I dont know how to fix it. Mine was only a couple weeks old, so I figured the beaks were just delicate, never thought it could happen to an older bird. And I only had about 6 birds in my brooder so it wasnt overcrowded or any of that, it was just plain weird and I cried and cried that they could possibly even do that to each other.
So so sorry you are going thru this. Mine never did it again but I separated them all out from each other, horrified at the little monsters. I did have one guinea in with the babies and I thought maybe that was what started it as they are a bit more pecky than chicks usually.