That was so industrious and thoughtful of you to hunt all these down, Robin. Thank you!
I remember the first time I saw in a medical paper that chicken hens were the only animal that experienced spontaneous ovarian tumors/cancers just like human women, therefore were studied to find cures for cancer in women. It really struck me that these hens produce an egg almost every day, at least the usual hatchery breeds most folks get at their local feed/farm stores, which is what I started with over six years ago for my first flock. Now, only one of those remains, a Buff Orpington, Sunny. She has not laid an egg in over a year and as far as I know, she just ran out-there is no abdominal bloating, nothing I can point to that is a problem, but of course, I wouldn't know for sure unless we necropsy her when she passes on.
The hatchery hens are pretty much saddled with genetics that are even more likely to cause issues-naturally, hatchery stock is not bred with an eye toward longevity. The last link Robin listed in her first post, regarding Maxie, was a fluke, a huge egg within an egg that was just too large to pass and dropped into her abdomen where it was inaccessible and was her demise, but it just illustrates one of many reproductive issues that you may see the longer you keep chickens.
We don't open up every hen who dies, but the ones we have necropsied have provided us with valuable information, and, in a way, comfort--we saw healthy livers and hearts (except for the one pullet who died of a heart defect as she came into lay) and knew that we were successful in providing a healthy environment and nutrition. Those just couldn't overcome bad genetics. They did, however, allow some of these hens to rebound over and over again before succumbing to their fates. Hopefully, these links will educate you or provide comfort in knowing that if you lose a hen to this, you did nothing wrong.
These threads may also be helpful to you.