Tac 20- hope you still view this, first as a former Iowaian sometimes these marks were on coyotes using metal flow tubes in that area but I have seen this many many times here.
Next time you get one part that hair where it is rubbed and look real close, sometimes there is what I believe to be some form or lice or something similar, the fur will be greasy in there. You will see them moving on the hide.Greasy larvae looking things.
I have watched coyotes repeatedly rub that spot with a back leg, the one spot they can really get going on with the back leg, right between the shoulder blades, my theory is that the spot right between the blades get hit the hardest by the back paw and the where down the fur there. I have seen them scratch for long periods of time with that back leg right on that spot.30 minutes at least.It is the only real good spot they can scratch like that.I'm thinking those whatever they are make then itch and that is the one spot they can get at
Just my theory, I'm no vet or bioligist.
Also I see you are from Esterville, we used to live in Sibley, had a nephew that played BB for Esterville on a state tourney team Todd Roede.
Do you know Tex Adams, I was an ITA director with him. If so when you see him tell him Hi.