One of our RIR's had a soft shelled egg this morning for the 1st time. She is 9 months old and has been a good layer from the very start. She gets layer feed plus crushed oyster shell is always available to her. She's acting the same as usual and looks healthy. I read an article in the Learning Center about possible causes of an egg like this and think I have narrowed the cause down to 2 of those listed: it's either the heat & humidity or she's getting ready to molt. There isn't a lot I can do about the heat & humidity other than keep her (as well as the others) as cool and comfortable as Florida can be in August. Is there anything I can do about preventing any more soft shelled eggs if she's going to molt? That is, if she is...I haven't seen where she's lost much in the way of feathers. The other 2 layers we currently have (another RIR and an EE) haven't had this issue yet. Here's a link to the article if anyone else wants to read it, it's great information: https://www.backyardchickens.com/a/common-egg-quality-problems
Sometimes it just happens. We had an EE give us a soft-sheller twice in a row from her perch, and it was neither heat/humidity nor a molt that was the cause. No idea on the cause, sometimes it happens. She went right back to normal egg production the next day.
I've been told stress can cause it too. I had a turkey that was laying soft eggs for a few days. I did everything I could for her and still no help. So I put up a game cam. There was a coyote at her coop two nights in a row! I killed the coyote on night 3 and she started laying normal eggs in a few days!