Originally Posted by Modshack

Typically that's the result of "tramlining". Cars with wide tires are particularly susceptible to this. Generally caused by slight road irregularities on heavily trafficed roads. Often you cannot see the road level difference. The tire track areas gradually sink a bit especially where there is heavy truck traffic. The tires nibble and fight side to side in this "groove". Some tires make this more noticeable.


Tirerack tech explanation here: Tire Tech Information - Tramlining: Coping with the Ruts in the Road