I agree with Planenut this is nothing to worry about, but I too have never found any definitive explanation about why it is so. My conjecture is that it's related to the cam timing of the original spec (emissions) cams. Those who have fitted aftermarket cam kits might be able to comment if they continue to experience this behavior with the new cams.
These engines aren't really happy when idling, and frequently running the engine for short periods, as opposed to getting them fully up to temperature is not recommended for any engine. In the case of the M3W, frequent short term running can lead to an air/oil emulsion clog in the breather hose between the oil tank and crankcase, leading to various maladies. And don't to forget these engines need airflow to cool, so running them up to full temp while stationary without overheating them is a dangerous guessing game.
The engine will live longer if it's left to slumber until the next time you can take it out for a proper drive. Just sit in it and make "vroom-vroom" sounds until the time comes
