Knowing that the waterer had gone dry one day last week (my bad) my theory was that a couple of hens might have become dehydrated while these eggs were developing. Very creative thinking, but not correct.
Today, we collected another wrinkled egg! No shortage of water this time. What's going on? Well, I needed some expertise, so I did what I always do when I need to know something. I googled it! Turns out wrinkled eggs are a real thing.
Basically, if the white of an egg is watery, it is harder for a shell to be built correctly around it and can cause an egg shell to be wrinkled. This is a problem of old age (we do have a lot of old hens) and some diseases, such as infectious bronchitis. Our hens eat a good diet and appear to be healthy. I guess they're just getting old … like me! The eggs are perfectly fine to eat.