I thought they did fire drills til they were blue in the face.
They do believe me I know I was a nozzle-man. They have the drills so you are ready. The drill doesn't prevent the fire though. Safety walk throughs do sometimes but still not always.
I'll give you an idea of a possibility. The vacuum cleaner may have had some issues in the motor winding and went unnoticed as it was put away. After being stowed it smolders and ignites the motor housing. The motor housing burns and other things begin to catch fire. Here is where it gets difficult and dangerous. If the compartment was air tight or close to it a good size fire could erupt then die down to a large smolder. It might even go unnoticed but in the mean time heat from inside that compartment transferred to the next compartment through the walls. I've seen it happen before two decks above a fire and another fire erupts.
Finally someone smells smoke. When you smell smoke on a ship you call Fire and then try to locate the space. You do not simply go opening doors. You place the back of your hand on the hatch or door. If it's hot your hand feels it and jerks away from the heat. You do not open that hatch or door until a hose team is suited up and the nozzles spraying or ready to spray. The sudden influx of oxygen into an oxygen starved fire can mean a serious flash happens. The fire literally goes from smoldering to hell in seconds in those conditions.
You have to always keep in the back of your mind that you do not know the oxygen level of any given space in a fire. For that reason after a fire is put out you still are in a fire fighting posture until that space has been checked for oxygen and explosive gas and the fire is overhauled.
The boat was in overhaul in the shipyard. The crew wasn't even onboard except for a below decks watch, a topside watch and whatever the nukes do in a cold-iron shutdown. Normal systems could have been tagged out or even missing during this period. This kind of fire would never happen at sea.