The number one reason is electrical problems, typically very old or very poorly done wiring. Typically short cuts taken. Also, electrical includes shorting of appliances. Number one problem appliance is toasters. Both toasters and toaster ovens. They should never be left plugged in when not in use. Other reasons are much like homes. Food left on the range in the galley, cigarettes in the stateroom. Improper storage of propane is sometimes a cause.
There are a lot of poorly maintained super yachts out there, super yachts that aren't so super.