How many birds do you have? In the coop birds need a minimum of 4 sq.feet per bird, in the outside run they require 10 sg. feet per bird, if they are over crowded they frequently will peck at each other, feather pull, and even kill & eat each other. Do you have too many roosters together maybe they are killing each other? Really need more details in order to help you. How many birds together, how many hens and roosters? Do they free range or are they kept together in a run etc?
Do you think one bird is attacking others, or a group of birds ganging up on some. Maybe you could borrow a game camera and set it up so you can see what is going on and which birds are doing it.