A few posters have suggested that shifting cargo might have punctured the skin. With a full aircraft, though, on a leg where there's plenty of cargo to boot, one would expect cargo compartments to be full. So, shifting isn't likely.
Those black and red bags, though, were probably stowed further forward and cascaded back when the original cargo or luggage was sucked out. What is that beam-like thing under the black bag at a 45° angle?
And looking at that remaining exposed stringer with the dark stain in the middle, doesn't the simplest possible answer just shout out corrosion as the most likely cause?