Mine did that and a tech knew exactly that it was idling to fast, twisted the screwback in two seconds and it never did it again.

 

yea sometimes the momentum of the rotating weight inside the engine from a fast idle will keep it going, and the plug can be heated which causes combustion instead of a spark. or you have carbon build up which after a long ride can cuase hot spots that still cuase combustion.

either way, seafoam cleans out the carbon really well and keeps it out, so after a tank of it whether it still does it or not you have eliminated one possibility. Techron or Seafoam are the good ones.