The combustion chamber temperature is too high and that can cause carbon in the cylinder to glow and continue to ignite fuel. This used to be quite a problem in the late 70's and early 80's when manufacturers where trying all kinds of things to get their engines to meet emmisions. Crappy fuel that ignites easily, too high of an idle speed, a carb problem, too much or too little ignition timing can all contribute to the condition as well as an escalated engine coolant temperature.