If it's only one tire, chances are someone might have unknowingly "curbed" that wheel/tire at some point and now it's running with excessive positive camber (the top of the wheel is farther out than the bottom) or excessive toe-in. A good large trailer shop can take care of a realignment, but like making sausage, you don't want to watch how they do it!!
Another possibility is belt shift on that particular tire. If that's happening, you might want to switch to the spare and monitor wear carefully to ensure that it's not edge-riding on the outer part of the tread.
Just a couple of possibilities.....
Rusty
