The only way to find out exactly what killed them is to follow @Eggcessive 's advice.All of them appear completely fine until they aren’t fine and it’s weird.
If one dies keep the body cold, not frozen and send to your state vet right away for a necropsy. They can rule out avian influenza, coccidiosis, Mareks, and many other diseases. Call them today before any more losses and get details about what to do. Here is a list of state vets:
https://www.metzerfarms.com/poultry-labs.html