well idk, but normally that is called "arching" and its usually from moisture being in the boots. Thats why they make di-electric grease to put in the boots of the plugs so they dont do this. a set of PRJ wires was doing this, and it turned the terminal on the coil pack brown.

idk, but check the insides of the boots. i have never seen my wires spark like that out of normality