That was my thought at first, that there was something causing a calcium absorption problem. However, after really going over everything, nothing really stuck out. I only have 4 pampered hens, no rooster. I actually clean their coop almost daily, and they always have fresh clean water. They have always have both grit and oyster shells available. I had their droppings tested twice and they are parasite free. The only supplimenting I've done is some dandelion greens, meal worms, crickets, mint, sorrel, and a very occasional pumpkin/squash.
After doing some brainstorming with several Veterinarians, the thought we had was that the layer feed that is most readily available to us, is the same commercial pellet feed that egg farmers use. The feed is scientifically designed to maximize egg production. In a commercial egg farm, they automatically cull hens at about 2-3 years old. It turns out the most common age for EYP is 2 - 2 1/2 years old. It is in the farmer's best interest to have a hen produce the maximum amount of eggs in the minimum amount of time. It's not advantageous for them to have hens that lay less eggs, gradually slow down, and then live another six or seven years without producing. So at the expense of the longevity of the hen, they force them to lay at a super high rate. This essentially overworks and eventually destroys their reproductive tract. Egg Peritonitis happens when the oviduct fails to catch the yolk either because of damage due to internal laying, or shrinkage of the oviduct - both factors of "aging" reproductive tracts. The yolk then travels into the peritoneum and an infection follows. This would be worse for a backyard chicken owner who can't free-range their chickens because their hens will be eating an even greater percentage of layer feed. So when we're thinking about the life span of a hen, it's only 2-3 years in a commercial or large farm, but possibly as long as 10 or more years in a small rural farm setting where the hens were more likely to be free-ranged, fed scraps, and allowed to actually sit on and hatch a clutch of eggs every once and a while. The most likely reason it seems like Egg Yolk Peritonitis runs through a flock is probably because there are many hens of the same approximate age, and they are all eating the same high egg production feed. It seems like a reasonable and logical theory, so I'm starting to do more research on alternate feed, and actually plan on doing more supplimenting with meal worms, forage greens, etc.