Here's my thought....even though the flex was improperly installed. I have seen where some localities require the gas service to be "bonded" or grounded to the electric service. Some localities do not require this. I have seen where there is a weak, loose or bad main neutral connection in the panel box or even the electric meter will cause current to seek a ground through the ground connection in the furnace which is of course hooked to the gas line. In this case it may have done that and arced to the gas line and burning a hole in it thus your "fire". The gas line of course can be a good ground. I also do electrical work and I can tell you many times I have connected or disconnected a groung wire from a main service to a water or gas line and got an arc! Any arc at all will signify a neutral issue in the main service.