It could be several things (unfortunately), or a combination.
There may be a protein deficiency which promotes eating feathers. Are the pullets eating the feathers, or simply plucking them? Feed them a high protein feed, with treats once or twice a day. Mine like raisins, tomatoes and raisins. Nothing fancy, and not too much at any time.
The chickens may need a larger area, with things to examine, a place to scratch, interesting bugs, roosting areas, etc. Are they bored? And is there room to stretch their wings and get up onto something instead of being only on the ground.
Are all the chickens participating in this, or is there one or two pullets that are bullying the others?
My four pullets are kept in a coop which is always open to a 5'x14' run. In the early afternoon they're let out to run/fly around and ruin my plants.
I keep their feeder full and they always have clean water. I'm surprised at how much St. Augustine grass they eat! They love to receive worms & crickets, wheat and other goodies.
I'm sorry the hens have developed this nasty habit. You may have to divide them and see if you can contain it somewhat.
Good luck! And please let us know how things go.
Carla