Lighty,

The HV output of coil seeks ground at the HV diode that is connected to common ground since that is paths of least resistance. It does find ground there there since it is in contact with the low voltage potential. Diode slams off and slaps it hard and it has nowhere to go but to ground at plug.

That is what is happening...it is a very simple electroradiant event like what the Gray circuit produces but on a much smaller scale.