I have seen frame webs cracked through before, and sheer ties cracked through, just thinking out loud after looking at that picture again that it would have had to have been multiple structural failure over quite a sizable area, all load paths are redundant, and a 747 airframe is built very strong. I dont ever remember seeing suitcases loaded loose on a pallet, crayfish ard mangoes yes..... but not suitcases.
The ULD's are usually half size cans, and do not have "doors" that seal, just a canvas flap that has pins to lock it "closed" if hold was full then cans would have been close to butting up against each other, I still reckon something would have had to "breach" the skin of the can.