For some reason this past year, my flock has been in a constant state of molt. It started in the spring, which was unexpected, and no sooner would one bird finish molting than another would start. My rooster Mars has molted twice--once in the early summer and again now that the days have gotten shorter. He's otherwise healthy and robust and is feathering out very nicely, but it was a surprise to be sure!
Yes, your hens can molt any old time, and this is about the time of year they normally decide to do it. Keep an eye out for new feathers growing in if you suspect molting is the cause, and consider offering them feather fixer feed to help give them a boost.