Lots of ideas on this "rubbed" topic.
There are plenty of things that cause hair loss. I've seen many coyotes here with thin areas on the top of their shoulders and neck. Most of these are caused by ticks and fleas . Just think of a dog when it sits on it's rear and gets that back leg going, scratching away around those hard to reach areas where it's collar would be.
Mange (micro parasite)is another problem , not a seasonal issue though , as they have it year around. In it's early stage,it does give a rubbed look. It get's more matted looking and then really bad crusty skin with little hair at all. Seen some that have skin that looks more like an alligator.
The mid to late winter (mating season) is what causes a lot of the "rubbed" hides that fur buyers don't want. Many things bring this on, the first being males fighting over females for breeding rights. A big one is the actual mating itself. The whole rump area and back of females and inner thigh and belly area of the males.
I've tracked hundreds of coyotes and fox over the years , I always check sigh along the way. I've seen a hair now and then in the thickest brush you can still crawl through. Field tiles and road culverts metal, concrete and plastic , will claim a few hairs from going in and out. More so on dry ones that fox will den in. Beds.... I have looked at plenty of them as well. Areas with plenty of grass and snow don't claim much hair due to melt and freeze from body heat. Years with lots of ice more and crusty snow will claim a bit more hair. I see this on female coyote and more so on fox that lay out in the wide open farm fields during breeding season.
As warmer weather (spring) approaches they will start shedding their winter fur . Plenty of animals do this, predators, deer, elk, bear, etc....
Just a few things I've gathered along the way growing up as a trapper and fur hunter.