What you are seeing is tire cupping. It seems to show itself more with aggressive bar tread(ie mud)tires. I have found that insuficient dampening to be a leading cause. It may be a bad shock, or not enough shock (this is one of the reasons Ford ran quad shocks on the coil front ends). Other things to look at are bad bushings, ball joints, toe in, toe in on turns (ifs vehicles), excessive front axle load (ie big bumper/winch/spare parts on a light duty front end), improper air pressure, and too soft a tread compound for the given axle load (I had this problem with my F150, the tire I wanted to run did not like the fact that my steering axle scaled in at just shy of 4000lbs).
These are just a few of the things to look for, but a good place to start.
Dan
"Sometimes one pays most for the things one gets for nothing. "
-- AlbertEinstein