Due to the electric current going through the air, the air is heated to 48,632 F°, (27,000 C°). The heated air is compressed from 10 to 100 times the normal atmospheric pressure, this causes the air to expand very fast which producing a shock wave of compressed particles in every direction, like an explosion, the rapidly expanding waves of compressed air creates a sonic bang. A couple of bangs one after the other is what we call thunder.
