It sounds like a molt. Most of the time a pullet that age would skip the molt and lay straight through until next Fall, but each chicken is an individual. All of them don't always do what most of them do.
There are things other than the days getting shorter that can cause a molt, usually things involving stress like running out of water for a while, moving to a new coop or location, introducing new birds to the flock, predator attack. About anything that causes stress can cause one, though a lot of the time, those are just mini-molts. This time of year, I'd really bet on the days getting shorter and a full molt.
I'd suggest you check your flock for mites and lice just in case. Check them after dark in case it is roost mites. Those only come out at night. Mites or lice are probably not the cause but it doesn't hurt to check. I'm betting on an old-fashioned molt.
