1]Excessive positive camber can cause your symptoms.
2]Excessive tow-in can as well.
3]Inadequate suspension dampening can also wear the outer edge of a tire.
4] It can also be caused by the tire being out of balance
If the edge is worn in such a way that it looks or feels scalloped along the edge, then it points to #s 3&4. (most likely #4)
When you get your new tires, check that the wheels are straight and be sure to balance the new tires.
The problem is obviously not serious, since 40,000 miles is a long ways for conventional light
trailer tires, and yours have worn relatively even.