Like a rim that's no longer flat or round whern there's no spoke tension prodding it to look true. Or maybe the Op is one of "those" riders who don't ride smoothly, weighs a lot and or causes their bike to receive more stress then typical.

To tangent to a rant- There seems to be this idea that a wheel should last for tens of thousands of miles, anything less and the wheel was miss built or just faulty. As wheels have moved from a wear item to a component, sometimes costing more then the rest of the bike, it's easy to understand this view. Yet the way a bike is ridden and the surfaces it's on hasn't changed for decades. I take issue with this wishful hope. It only takes one slight hop, one thumb sized rock, one ripple (not even a pothole) of the roads surface when the rider's weight placement is not "correct" to cause the rim to become deformed. Many times we don't attribute a riding incident as the root of the problem. So I usually look past claims of mileage and instead focus on the rim's condition. When the wheel is laced up, tensioned and trued The rim will look fine. That's what spokes can get you (and generally the more the spoke count the less a rim condition will be noticed (until...) Andy