Hello-
Are your chickens new layers? Occasionally they will have some thin shelled eggs that break. My layers are 1 and 2 years old, and this even happens to them occasionally.
If you start seeing alot of thin egg shells, then it's a good idea to supply them with a container of crushed oyster shells that they can eat if they want it.
This happens from time to time.....Just keep your eye out.....