Geoff,
"What is very interesting is the fact that you can see items of luggage exposed. Why that is interesting is luggage is not just packed in the cargo hold loose. It is packed inside air cargo containers at the terminal, and then loaded as a container into the cargo hold of the jet."
I think bulky items get loaded loose. Reading the passangers observations, that the floor caved in, the air was going only one way, no explosion only a bang.
I think its corrosion, many rivets have broken. If the rivets were healthy, some would have stayed and torn the sheet.