i think they knew it was gonna happen. Why have the camera set up otherwise? And the guy standing near the truck doesn't look surprised at all. Also, you hear the sound of the arc, which means the mic was on. You don't hear any "HOLY SHIT!" from the cameraman, despite the fact that he's not a pro (notice how the camera is tilted about 10 degrees to the right) so it doesn't seem to have surprised anyone
my guess is they were making a "this shit's powerful, don't mess with it" training video.