I've had the same happen to one tire on our trailer, as well. Oddly enough, the wear was on both shoulders of the tire. Typically that means a tire was run underinflated, but we run our Sailuns at 110 psi all the way around. I also constantly monitor them with a TPMS. The bearings are serviced and in great shape. It was just weird.
Last summer, between having a tire plugged (it's now our spare) and having our wheels replaced by Tredit (due to two cracked rims in a row), we ended up moving that tire to another position and the former spare was placed into the wear position (front axle, door side). I'm now watching the tire placed into that position fairly carefully.