I have a hen that seems to be over-grooming herself. It's bad enough that nearly all her feathers have the ends broken off, and some have broken off more than just the ends. I've checked her for mites and other pests, but found nothing. (I treated the whole flock anyway, but it didn't help.) She's definitely doing this to herself, it's not pecking from another bird. I can see her breaking her own feathers off when I watch long enough. I've done everything I can think of to remove sources of anxiety, but I'm running out of ideas. There's a second bird that is also overgrooming, but not nearly to the same degree.
Does she eat the bits she breaks off? If so, she may need more protein in the diet, or some other micro-nutrient. Even if she's got a good diet, some animals need more of something than those around them do (I need extra magnesium - don't know why, but I can tell if it's low and I'll be eating the same thing hubby and #2 son are eating!)
We tend not to think of this with chickens, but some individuals also struggle to absorb nutrients even if the diet is good. In chickens I've heard of their digestive tract being slightly damaged by coccidiosis. In this case, something like a natural Apple Cider vinegar (diluted in water) or some version of Kefir might help? If it did help, you'd have to observe to see if once "good bugs" are in the digestive tract they stay and colonize, or if she needs a dose weekly or more or less frequent.
Or is she molting, and confused about the prickly feelings? My hen Blackbird got psychotic when she was molting. She kept thinking the others were hurting her, squawking at them and running away. They could see she was nuts and she lost her position at the top of the pecking order.
Got it back after the molt was over. She's a natural leader!
But enough about Blackbird. Are her feathers dropping? Do you find them around the coop/run/nearby? It's just a thought.