Check your feed--smell it and look for mold and an off-odor. Mold can kill your whole flock or make them very sick. Botulism from eating animal remains, maggots, or rotted vegetation can kill very quickly. This can cause the neck feathers to easily be pulled out. The paralysis starts in the legs then the wings, and eventually the neck becomes paralyzed. Old lead shot found in the ground or old paint chips, chemicals, rat poison, and many other things can cause similar symptoms. You really need to look around your yard for dead animal or fish remains, block off compost beds, and I would get a new bag of feed just in case of mold. Mold can occur overnight in feed--I have had that occur, especially in scratch grains, but also in layer feed. Sorry for your loss. Remove any dead birds right away, and burn them or bury them very deeply in case of botulism.