I've had this happen a few times. I took this explanation from here.
"Declining day length: Hens are sensitive to day length, and particularly to the direction in which day length is changing, when it comes to laying eggs. Declining day lengths discourage egg production.
It is not unusual for a flock owner to have hens go out of production in the latter part of summer and in the fall because the days are getting shorter."
There are a bunch of other reasons on that site but that seemed the most likely to me. - timjeffries