The texture of a road surface is what determines if a car will tramline. Tramlining can be magnified by a car alignment and tires. Neutral toe will see more movement. Toe out and you'll fight to keep the car on the road. Triple so if the toe out's in the rear. Certain tire tread patterns and some tire compounds will want to tramline more than others.

Ineffective power steering doesn't cause it. But boy would you get a workout if you had to fight it without power assist.