Anytime I read of workers killed in explosions for which no cause is known, I try to read between the lines.
This plant made carbon and graphite or used them. They are both excellent conductors and frequently become airborne to settle in electrical parts. Usually these materials are not combustible themselves but they can lead to arc flashes if the work environment is not kept clean and regular cleaning of electrical equipment is not done.
Think about housekeeping as preventative insurance on arc flashes and explosions.