No tire is perfectly assembled, so the belts are always a tiny bit off true center, and always a tiny bit off perfectly radial. The rubber is never perfectly even consistancy nor perfectly distributed, so as the rubber wears off the tire, the compensating weight added becomes too large or two small as the weight distribution of the tire changes.

Add to that the fact that tires almost never wear perfectly evenly, and there to go -- out of balance.

Impact damage makes this worse, so do bad shocks (cupping) or misalignment of other types, or heavy cornering.

Just a fact of life -- get them rotated and balanced every 5000 miles or so, and they will both last longer a ride better.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!