You could have a dominant hen mounting and doing this to the others. I would spend time observing my flock. To me it looks like tail feathers could be being yanked on and broken off, too. Sometimes flock mates do this because they’re bored, not enough space, she’s lower on the totem pole, overzealous when grooming, protein deficiencies, or unknown reasons.
As sometimes this happens at roosting time, a WiFi camera in your coop can help you spy on them at night if your WiFi reaches your coop. If it’s a dominant hen being too rough or bullying, pinless peepers can help, but I’ve never used them. I don’t think they should be used too long as they can case injury and irritation.