I'd say it's a mini molt unless she has been picking her own feathers and eating them or another bird picking at her.
Chickens usually molt once a year starting in autumn. However they can have mini molts at any time. They molt fluff as chicks when they start to feather out as an example.
It's natures way of replacing old or damaged feathers with new ones.
However, molt can also be caused by stress, improper nutrition, light on in coop at night, overcrowding etc...