Lacrystal, I know what you mean. I had a Marek's nightmare this past summer. I did what you did, waited until I thought their quality of life needed to be ended.
Like Poseygrace, I would think botulism (quick death from symptoms to death), or a vitamin deficiency. Maybe coccidiosis. Maybe...
Hi. It's highly unlikely that it's Marek's. But here's some ideas
Is it too hot?
Is it the food? I would get another bag and see.
Then make them a wet mash with it, to see if they eat it. Sometimes the crumbles are too big.
Botulism would cause a symptom like that, and they would die pretty...