I'm sorry for your loss! I'd recommend getting a necropsy done. I'm not sure how much that would cost you where you live, but for me here in PA, there's a lab that does poultry necropsies for $30. That's the only real way to tell at this point what happened to your hens. Then you could possibly prevent it in the future, depending on what it is. Best of luck to you and your flock!