When you let the engine sit for a while the oil drains from all the crevices. So when you start the engine it's not lubricated well for the first couple minutes while the oil gets pumped back in. (I'm not sure how it works with a 2-stroke, I assume it's the same with the mix).
That makes sense. However, I remember hearing somewhere that engines can seize up after a while if they don't get started.