This is a tire that is worn from a toe angle that is out of adjustment. Many times people assume that only the front end of a car is necessary to align, but the rear end is just as essential. As in this case, the right rear toe angle was out by about a tenth of a degree. For how long? I don't know, but what I do know is that the end result is really worn out tires, as well as lost fuel economy. Now some other factors could be involved here, such as worn bushings or struts, to cause a similar problem, but if you look at the way the wear pattern is diagonal gives away the toe angle issue immediately. The other thing to check for when you see this is worn bushings. Worn bushings can change the angles of your tires as you drive down the road so make sure that when you take your car to get an alignment you have them check you suspension and steering components as well.