What could be causing this tire wear?

Toe-in.....

 

Here you go. Just because someone checks wear parts, or claim to have done an alignment, does not mean it was done right. If something is "within specs", it probably still isn't right, and no one wants to spend the extra time and work to shim or bend an axle to get it right.

http://www.procarcare.com/includes/content/resourcecenter/encyclopedia/ch25/25readtirewear.html

 

Did they give you a printout of the before/after specs? That looks like toe-in to me. If it's only on 1 tire, you may have a problem in the rear as well. Lets say for example you have a slight rear misalignment that cause the truck to steer left, you when your going straight, you are actually steering right slightly to compensate (unknowingly). In in a case like that, the right steer tire is going straight, while the left has excessive toe-in. That's how you get 1 tire worse than the other.

Most of the time, when you see signs of toe-in on one steer and tow-out on the other, it's a problem in the rears. We have found on our trucks, the best steer wear comes from setting up the front drive parallel to the steer axle, but setting the right side of the rear drive back 1/32". This puts a slight touch of rear steer to the right to counter the crown of the road. We set toe at 1/32" toe-in.

Feel for feathering on your steers. Feathered in toward the frame means toe-in, so if your hand sliders smooth on the way in, but rougher on the way out, you have too much toe-in. If both tires are toed in or out the same, then it's a toe adjustment. Of they are different, you have something else going on.