Likely solvents and lubes and paints on the hangar deck.
Just out of routine maintenance, no ammo aboard. Very little fuel.
They were likely preparing to paint. And a ship that size requires a LOT of paint and cleaning solvents and primer.
All oil based. And highly volatile.
LA Times says 11 sailors with minor injuries, no deaths reported, all sailors evacuated and accounted for per the Navy spokesman, thank God.
https://www.latimes.com/california/story/2020-07-12/fire-uss-bonhomme-richard-naval-base-san-diego
Paint, dried and on the bulkheads is fuel for fire.
Add in any aluminum superstructure.