It's not necessarily just a poor job of preparation. Some grain changes dimensionally with the moisture content of the wood. I've finished stocks that were sanded smooth and dead flat only to have the surface change in that fashion months later. I believe top coated finishes (poly & lacquer) after dry sanding as factories or high volume shops are wont to do, are more prone to it than penetrating oil finishes applied with wet sanding.
In fact, in a couple instances where it occurred for me I wasn't able to completely eliminate the phenomenon with a refinish.
