I have never seen a glow engine start on fire so I would guess it was a spark from something electrical?? When I am flying a gasser I do have a fire extinguisher right next to my start up area but I don't bother when all I have is glow powered planes. I'm the only person I have seen in a long time that has an extinguisher on hand with a gasser but I think some of the guys have them in there cars? Fire and glow is a very strange happening?