One or more inner ribs worn below the level of the adjacent ribs circumferentally around the tire.
Lack of shock absorber control in some suspension types, mismatched tire sizes and/or inflation pressures between duals, loose or worn wheel bearings, assembly non-uniformity such as improper bead seating and out-of-balance condition,
aggravated by high speed empty hauls.
If not worn excessively, continue to run. If wear is excessive, retread.
Diagnose mechanical problem and correct.