this is often quite common with chickens. I wondered was there things we could have done but even after a full inspection of the carcass there were no signs of mites, infections or even trauma, though the night before I did close up the coop because the temp dropped to almost freezing. In the morning she was her usual noisy self and fighting the other hens over sunflower seeds and when I got home from work I was only greeted by five hens and there she was in the dusting area half buried in the dirt.
I let the rest of the hens out so I could remove her and clean out any feathers or debris where she was. Still puzzled I did some research and found this was not uncommon and though we did not process the carcass for consumption, due to not being able to tell how long she had been dead and it was a rather warm day, I did inspect her wings, vent, neck and head area because after the very first cold spell at the start of the winter she was not looking well. She was disoriented and shaking her head a lot. So we brought her in to warm up and let her rest in a dog carrier. After two days she went back out and seemed fine, but as winter set in we started only getting 5 eggs on average, so one of them was not laying (we never forced laying and let the girls do their thing) but on occasion we would get one extremely large egg, jumbo +, often it was wrinkled shell and extremely think walled. Just over the past week we had four occasions to find rubber shelled eggs, so we watched and egg production returned to normal. Then poof day after six normal eggs she died.There could have been other signs, like she never roosted with the other hens, she would go to a higher spot in the coop and not share body warmth from the others. She often was last to eat but had a bit of a temper with the other chickens. She was not a runner and barely followed the flock when they all saw one of the others getting something good to eat, or so they thought. She hated worms, while the other girls would relish at the opportunity to get into the compost bin to get the worms, she did not. She hated to be picked up and would go to great lengths to get away.
In the end we still have no idea how she died but are a bit concerned and will watch the other girls for the next week or so to make sure they are fine. I did run across this article from Mother Earth News on sudden chicken death, all seem plausible based on what we observed.
http://www.motherearthnews.com/homesteading-and-livestock/sudden-chicken-death-zbcz1404
