In my experience this is usually scarring of some type. I’ve seen it pick up this time of year and the dark spot gets much wider once the boars start fighting .
Foot traps. If you look at pictures of finished coons alot will have a black line about an inch wide and 3 or 4 inches long. It will be from behind the ears down to the front shoulders in a straight line down the back.
I would go with the scars. Maybe the coons are caught in the same area, these coons coming out of the same den getting rubbed? Sounds different to me if it isn't a trap or snare causing it.
Ive never seen it on fall or mid January coon but about February most have some. By March 1st is very pronounced. I’ve been told this is associated with rutting. I don’t know coon biology well enough to argue this point. It may be where the coon start losing their winter coat first, also.
We call them scratch backs. I believe It's ticks and other mites gathering In that area and the coon are scratching at them. I have seen It happen In Dec coon. In fact I sold several coon to groney a while back and 3 were scratch backs and the other were Ok. The scratched backs brought $6.00 while the other ones brought between $18.00 and $12.00. You will get docked for those scratched back coon.
...will have a black line about an inch wide and 3 or 4 inches long. It will be from behind the ears down to the front shoulders in a straight line down the back.