This interesting explanation came in from IJ of Bean. Quite how much 'stepping' will occur will depend on the rear brake bias, so the more there is, the more 'stepping' there will be:

“Rear tyres on front-wheel-drive cars will wear the tread blocks into a saw-tooth profile, as the tyres only see braking forces. This can be noticed by running your hand round the tread one way and then the other. This can produce some very alarming noises. I thought the wheel bearings had failed on my Vectra when I first experienced this many years ago. The solution to this is to swap the wheels from left to right across the rear of the car every 5-10,000 miles, unless the tyres are directional, in which case they can be swapped with the front wheels, keeping them on the same side of the car. The front tyres are subject to braking and driving forces, so do not suffer in the same way.”