Well, AFAIK this hasn't been formally stated. Demolishor is seen getting his head cut off in the ShoWest footage. Credit goes to user 'scalan15' for suggesting it. Here's the proof. Here's a crop of the scene of Demolishor running through the overpass. Here's the mysterious 'exploding thing' scene from the ShoWest footage. The head is upside down, and the fireball is where Demolishor's neck would attach to it.