From looking at the video I would have thought the fire weakened the tube and then it imploded. I have seen something similar happen due to an over acheiving fuel pump on a big Simjet once, scared the doo-ha out of me for sure. The thing kept on burning, we used up two extinquishers before we got it out.
Thanks for giving us the real cause, and sharing the video.
Have seen several incidents due to these weak pipes letting go.