They could be ready to start their molt, it could be from the decreasing daylight. I have many non-hatchery type birds that quit for most of the winter. I got 11 eggs yesterday from 47 girls, all of laying age. I have many that were sitting, and many that are beginning molt. You could also worm them and check them for lice/mites.
I started worming them today, just to be on the safe side. I did earlier in the year, but I have added a "few" new chickens to the flock, and they may of may not have had any worms. I haven't seen any signs, but like you said, there aren't always signs. They also free range quite often, so, who knows where they may have gotten them.
They are black and white pied and still actively laying. Maybe they will encourage the others to start.

