I'm not saying that hot work can't be the cause, but that there's got to be some gross negligence to go along with it that cannot be prevented by ship's force unless we stay with them 100% of the time.
Try to light a bunch of construction material on fire with a few stray sparks, let alone one that instantly grows beyond the capability of a fire extinguisher. It doesn't work that way. You need prolonged exposure or some kind of oil / fuel, as shown in this video, to get it going (and note he still has time to calmly put it out before it reaches flashover).