In Chapter 22, what does Maven say caused the explosion in the ballroom on the night of the attack of the Scarlet Guard?

Mare asks Maven what he knows about the bomb that caused the explosion in the ballroom on the night of the attack of the Scarlet Guard. Maven tells her that it was not a bomb. There was an explosion, but it was an accident. A bullet punctured a gas line in the floor, and when Cal's fire hit it, it exploded