Entirely speculation here, but this is why I don't like lifting keel sailboats. I'm guessing that it was raised while at anchor which made it less stable.
Not necessarily a problem, but bad weather has to be handled particularly carefully.
The lifting keel was definitely there so he could get his look-at-me boat closer into harbors.
The keel, lifted or not, should be providing enough righting moment to bring the boat back upright in a matter of seconds. Unless…
Unless the keel breaks off, then you’re done for. But the divers said it was still attached, and no mention of any damage to it. Armchair QB, but I’d say this indicates that the keel never came out of the water - it was not tipped over far enough for that to happen. To me, this had to be open hatches and/or broken windows letting in massive amounts of water quickly, with the potential complication of a huge amount of furniture not secured and permanently altering the balance once it tipped (speculation!). The mast breaking off could also alter balance enough to cause problems.
I’ve been on a sailboat that got put on its side by big wind. It’s terribly frightening, and then a few seconds later you’re right side up again. It is really, really hard to sink a sailing boat if the hull is intact!
The mast was the tallest aluminium mast on a yacht, it snapped and fell down which upset the balance of the yacht.