The volcano is exposing or changing an effective electrical charge in the earth, increasing the electric field to the point where it exceeds the dielectric strength of air. This results in the air dielectric breaking down and dischaging the giant “capacitor” in a lightning bolt. I don’t see it as a great mystery, given that magnetic materials are moving in a magnetic field creating electric currents and charges. Any electrical engineers or meteorologists want to chime in?
Not when you explain it as well as you have. For those of us not so well versed, it's an amazing sight.