A look at the metal deformation around the edge of the breach gives you some indication that there was possibly a rapid expansion of gas or vapour. The short sections of stringer that remain attached to the folded skin at the top of the photo clearly show signs of sudden outward pressure, witnessed by the torn edges. The skin around the edges of the opening is clearly sharply torn with an outward fold around. Now I wouldn't even offer you a guess ( that's why we pay the ATSB to do a job) but as someone who has spent some quality time on demolition ranges, an uncontained explosion, or expansion, inside a vessel will find it's way via the weakest point to the outside as quickly as possible. Which is why when objects such as cars get sent to a better place, all the fragmentation and remains are sharply torn in the outward direction. There are plenty of industrial engineers and likely metallurgists on the site. The damage to the stringers, the skin and direction of folding should give these guys hours of discussion.
Anyways, despite the unfortunate incident ( I'm sure Geoff Vader still hopes it might buff out!) it stands as a testament to the ruggedness of a design that first flew forty years ago