Right, I live near Bismarck and fish some 60 times a year, at a minimum. In the spring those trips are fairly short, about 50 mile roundtrip. By the end of summer I am doing 170 mile round trips on average. So I figure I'm in the range of 6k miles a year on my trailer tires. If you smack a pothole, porcupine (been there, done that), drive too fast on washboard roads, etc, etc and you get a lot of opportunities to screw up the trailer's alignment and it's easy to see on the tire wear.
My Toon's trailer with it's double axles and small diameter tires is really susceptible to wearing badly. I suppose I only got about 10k on its first set before they needed to be replaced.