That is not an uncommon thing really, you should never leave the shop right after welding or making sparks, I always wait atleast 15 minutes.
I have never really started a fire but I did one time, while grinding with a 4" grinder, set off a whole pile of firecrackers, the sparks set them off, scared the living daylights out of too, threw my grinder and ran, it took me a couple minutes to figure out what happend...and get my heart back to a normal rate.
I still think about it every time I pick up my grinder.
welder19