The Lusitania was carrying small arms ammunition which is incapable of "exploding".

Two other theories exist for the secondary explosion:

Ballard's opinion that coal dust sank the Lusitania, she was running 'light' when she was torpedoed, the torpedo hit her and caused a large cloud coal dust to form. It was that that exploded when ignited by a broken electical light fitting and accounted for the second explosion.

Others think that the second explosion was caused by ruptured high pressure steam lines.