hmm
could be a host of items , poor quality gas , carbon buildup thats retaining heat in the chamber sufficient to provide ignition , what carb and what condition is it in ?? , idle set up that may be too lean , restrictive exhaust ,,, just a few places i'd look . Something is causing it to retain enough heat for spontanious combustion without an obvious ignition source. A good flush of the cooling system might help as well or a general check up of the cooling system might be in order. There is a passage near the plug side of the chamber that might have some corrosion built up thats might be acting as a thermal barrier and not allowing heat to escape to the cooling system out of the head.
My dad back in the day would always bring a car just above idle a couple seconds then shut it down to reduce this condition.