Methinks someone's eating the ends of the feathers. Not surprising, since the Wyandotte breed is noted for being a bit aggressive. You might want to increase your coop/run space, or get something to distract them. For my purposes, a rooster is a very good distraction, but I have a lot more hens than you do.
Moult would result in losing lots of feathers, but you would see new feathers or pinspikes (except under factory farming conditions)
Mite damage can result in shredded feathers, but it's generally not localised to the back like that.
A rough rooster would have feathers missing from their shoulders, and they would very likely have a bald patch just below the comb.
Some chickens have fragile feathers, but that's usually most common in Lavenders, Frizzles, and Frazzles.