I'm not sure! It kinda looks like feather plucking to me too. I have never had experience with mites before and hope I don't. How much space do you have? I read somewhere that you can use a paper towel and wipe your perch with it at night. Not sure if that works I've just seen it. And you can do...
Hi I don't know what would cause it but something that you could do is put a chicken cape on their backs! We have had this problem before with one of our hens mounting another hen. We ended up getting a chicken cape which you can get from Etsy and then she ended up growing the feathers back...