Thread strip coming off tire, help me find the cause?

The second picture looks more like something hung up on the trailer and scraped it, or it rbbed a crossmember while dragging it with out the bags inflated.
As far as the cups, just sliding the brakes a few feet will sometimes start the process.
Under inflation of one tire will always start them down that road too.

I have a log trailer that always wears the same 2 tires out 3 times as fast as any of the rest of them. I am thinking that at some point in time, or maybe it was built with the spindle just a hair off.