Likely happening during roosting.
I would still give her a look over for lice/mites - it may be another hen is grooming her for bugs. It may also be just grooming her for no reason at all. Possible whoever is nibbling the feathers just has a bad habit or ....? Chickens can be weird.
For...
Do you have a rooster?
If not, that's ok, I would say whoever is roosting beside her is nibbling her feathers. Feathers are intact, just broken/worn/nipped. She's not molting yet.
If you have others that look like that, it would be a good idea to check them over for lice/mites too, but...