Tire causing alignment issue

Sounds like you did not get 4 exact identical tires. One tire is slightly bigger (in diameter) than the others, causing the pull. Also, most roads are sloped to the right, which causes vehicles to want to drift to the right (ditch), and that constant fighting of the wheel could cause tires to wear uneven. Rotating a back tire to the front or front to the back is a good example, as the steering tire is the one always fighting the resistance to go in the ditch, causing more wear on it.

My guess is, the tires are not identical in diameter. One is ever so slightly larger in diameter than the others, causing the pull. Have your tire shop measure each one for you.