I'm in no way a professional, nor do I have much knowledge about cars, but maybe the plug just has losened (spelling?) a bit, and that's why there might be a bit of a leak, and as the engine is starting to warm up, the leaked oil/petrol mixture is starting to evaporate ?
Or maybe the plug just has oxidated a bit where the cable from the ignition comes onto it, and therefor it heates it self up because of the higher electrical resistance ?
But that are just vague guesses