Chickens dying at alarming rate.

[aart]

All of them appear completely fine until they aren’t fine and it’s weird.

The only way to find out exactly what killed them is to follow @Eggcessive 's advice.

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