I had the same thing happen to my tires,right side only. I had one replaced in Butte MT at Wallins up in the old part of town. This fellow said that the right side will always wear faster than the left because of the road camber,more weight on the curb side than the drivers side. As the trailer travels down the road it is not running level,always about 2inches or more lower on the right side because of the way the roads are made. He also said that on heavy trailers sometimes this will cause the belts to shift causing a rounded look.
Now, I'm not a tire guy,but this seemed reasonable to me. He also said to rotate the tires side to side every few years depending on the wear. I replaced mine and have had no further problems.
