If they were about half grown and were the smaller ones then it sounds like genetic like the others said.perhaps they weren't doing good and that's why they were so small?
My only experience with them just dropping dead at mostly grown was poison (Although I did have a few scissor beaks i culled because they couldnt eat enough). It was curly dock that was seeding next to the coop, the smaller ones thought it was easier to eat the seeds through the coop then compete for the food. I saw them eating it, but just thought "oh cool, they like that, I guess that's a good food source then". Then they got withdrawn and tucked their shoulders up, seemed quiet and tired. I thought they were being bullied and tried to keep an eye for injuries or who was doing it, then they started hiding more until they just died. By curiosity I looked up the plant after they died and found it is poisonous to ruminating and crop animals, with those symptoms. I rampaged and pulled it all up, and no more deaths.
