I would lean more towards what bft is talking about. It makes sense if you think about it not doing it over night when you crank up in the morning. The oil has gotten thick and not moving, but after you drive it and the oil thins down from getting up to temp and it slowly passes by the seals the next time you crank it some has seaped through and is warm so that when you crank it, its warm enough to burn off quick and your smoke comes from that