I was getting several like that. What I read seemed to say that it is caused by watery egg white making it difficult to put a shell on. I switched to Purina’s omega layer and cut back on treats. It did get better but then she stopped laying all together. I hope just for the winter because the other suggested cause was infectious bronchitis although she hasn’t had any other symptoms.
We got a really strange egg like that a few months ago - we called it the alien egg. Not sure what happened - I know which girl laid it and she was perfectly fine and healthy. It was extremely cold when this happened, so I thought maybe she put more of her energy and nutrition into staying warm vs the egg that day. It never happened again... Curious to see if you get that again or if it was just a random thing...
Great link provided by @Miss Lydia. Sometimes it just happens for a variety of reasons - most frequently it seems at the beginning or end of a laying cycle. As long as it does not become routine, there is not a significant problem.