Have you seen the "far-side" comic with two goldfish on the carpet staring at their bowl which is on fire? The one says to the other, "we escaped the fire, but now we are equally screwed."
You know catching an aquarium on fire could make quite a story! 8)
My personal guess is that even if the filter ran dry, (which I don't think it could - not a HOB
, at least) it would not start on fire. A dry power filter would simply not turn, or maybe grind a little, but would not spin so fast is started a fire. I'm guessing that the electronics inside shorted out, causing a fire. Perhaps due to a defect in the keep-water-out seal.