It is hard to guess what was wrong with the chickens who died. It could be anything. Some of their symptoms sound like coccidiosis, except that usually affects younger birds, and they don’t eat and drink normally. I would get a necropsy by your state vet if you lose another to look for a cause of death. If any become lethargic, I would either get some droppings checked by a vet for coccidisois, or start some Corid. Sorry for your loss.
