Lusitania was probably a coal dust explosion but the UK didn't really help matters about the accusation that she was carrying ammunition by using her as a depth charging target post war, wrecking the wreck even further.
I'll definitely agree with Ballard about stuff like that, the man knows his wrecks. But there's a lot of competing theories, and history is never made of a single event but a nice little tapestry of charlie-foxtrots woven together. Data from the Royal Artillery Regiment confirms that the munitions she was carrying were actually live, powdered and fuzed, and the magazine was just forward of the coal bunker and also in the apparent strike area of the torpedo. Ballard's theory goes with the torp hitting just aft of the magazine, since he found no torpedo damage at the forward end of the magazine itself (but doesn't rule out a strike at the rear, or on the bulkhead between the bunker and magazine) hence the coal explosion.
There's also powdered aluminum that was being transported as well, which aerates into a fuel-air explosive as well if sufficiently disturbed. Worth consideration is a bomb aboard the ship that detonated sympathetically - there was the Black Tom explosion caused by German agents, and just after Lusitania sailed, three German spies were arrested with a camera aboard the liner. Their mission was never learned, but the thought is that a bomb was placed aboard to deal with the illegal munitions shipments to the Brits. Wouldn't be the first time, and they were caught onboard.
I suppose, with all the salvage attempts, the damage from the depth charges, the fishing gear, and the overall structural failure of the ship plus the rapid state of decay, there's not really going to be a way to find out what happened.