W211: E320 cdi - what causes rear tires to go bald in the center

took my winter tires off today to replace them with dubbed tires (tires with spikes) and was irritated to see the rear tires had a one to two inch almost bald strip in the center, the edges were just fine with good grip

so does that mean the tires were over-inflated or under-inflated or something else ?

cheers
niall