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If your chickens are in their second fall, they are likely starting to molt. Notice the pin feathers coming in on the bald areas. It can't hurt to dust them, though.
Check their necks, under the vent, and top of heads for pin feathers poking up through the feathers. That will be verification they are in molt.
Couple questions.
Do you have a rooster? The feather loss looks like an over zealous rooster who has a couple favorite ladies.
Have you seen any mites on your chickens?
It sounds like a rooster's work if you have one. You can get a hen saddle to keep him from damaging her back more.Do you have a rooster?
I would venture somebody is roosting beside or behind her and giving a good pluck at night. You could try an apron to discourage.