Also, a few years ago, Gordon Lightfoot changed some lyrics when they found the main hatchway did not give in.
I met a guy once who dove the wreck. He thinks some of the hatch bolts weren’t tightened all the way and water came in to the hold, making the ship heavier and harder to control.
That seems to me to be the most likely cause.
However, anytime you cut a ship in half and add an extension, the hull weakens.