Hens keep getting sick!
Worming is something that needs to be every now and then, throughout the lifetime of the bird. It's not a once and done sort of thing. As they roam, scratch around, and consume bugs, they are constantly being exposed to internal parasites. Eventually the parasite load becomes too high for a bird to stay healthy. A sudden fluctuation of temperature, combined with a high parasite load could stress a bird to the point of death.