Couple of good photos of the ship, the first one shows the upper deck area which due to the height of these ships is rarely seen, but I really like the second one with the guy in the foreground paddling past on his surfboard?, and paying no attention to the wreck.
Having spent nine years at sea as an Electrical Officer between the late sixties and mid seventies, and experienced a number of emergencies, it gives me a bit of an advantage in looking at the possible causes of the list. I saw the press conference on Sunday and for the Captain and Pilot to have come to the decision to reverse course and put her aground on Bramble Bank as quickly as they did showed excellent seamanship.
Since she had already come down the Solent without listing she probably wasn't inherently unstable, and even if she had been, and started to list when turning to starboard (right) when rounding Calshot she would have tilted to port since ships lean towards the outside of their turn. I think that the two most probable causes are either that a mistake was made when pumping ballast and that it was pumped out of, instead of into, the ballast tanks thereby making her unstable; or, as the engine room and other machinery spaces contain a number of very large pipes carrying seawater for cooling purposes, one of them suffered a fracture or a joint failed. The latter happened to a ship I was on and we very nearly had a flooded engine room, which given that it was a loaded tanker, would have probably resulted in her sinking.