Yeah I live in Northern US and about 1/3rd of my flock molted in winter for the first time. I can't think of any other reason except for as you said, the warmer fall.
Chickens don't necessarily molt in fall, mine always used to, but this year I've had about ten of them go through the molting process in the last month or so. Some went through full body molting, others had it just like these ones, with just the neck and chest area losing feathers. But they all...