Psgn side tire is wearing causing some tread blocks to wear lower than the others across the tread face of the whole tire...no issues w/ driver side or rears.

 

Cupping of tires is rather common on dually trucks, simply due to lack of rotation. That said, if the cupping is only occurring on one side, then a worn suspension component is commonly the culprit. Check your tie rods and ball joints. Chances are it is either one.