The heat of the arc rapidly expands air (like a lighting bolt), the heat of the arc also vaporizes the metal conductor, and the vaporized metal expands rapidly,

The heat of the arc rapidly expands air (like a lighting bolt), the heat of the arc also vaporizes the metal conductor, and the vaporized metal expands rapidly, and enormously. It's basically a small explosion. The hot metal then ignites in air adding even more heat to the explosion.