[wingnut60]

Stiff sidewall tires will help, but keeping the air pressure at max listed on the tire also will help. This is normal sideward force placed on the tire/wheel because the trailer is pivoting on the front wheel and the rear wheel moves sideways more so then forward/backward, causing it to slide rather than roll. As I said, if you do some sharp turns, the back tires will tend to slide sideways rather than roll. Leaving a tread track is also normal on flat pavement--on gravel, the rocks will move allowing the tire to slide.
Good eye that you noticed it. Keep watching all tires for uneven wear, and stay away from sharp turns that cause you to hit a curb--much harder on the tires than scrubbing.
Joe