2. Arson to cover up espionage. Again so much for security.
3. Saving face for being plain stupid that they disengaged their fire suppression systems and allowed work to continue without a fire watch. The Navy taking its time looking for a scapegoat.
4. The Navy looking for a conceivable set of circumstances to release to the public to explain self spontaneous combustion of a multi-billion dollar warship and blame nobody.
I am leaning to 2 and 3 myself.