Originally Posted by Bern Hanreck
300 year waves ( rogue waves ) and colossal releases of gases from seabed ( eg methane ) so vessels no longer displace their own weight of seawater therefore sink......
Even simpler causes - like sudden shifting of cargo or bow doors not being shut properly can cause capsize.
Quote from sinking of the Herald of Free Enterpise:
"The ship had capsized so quickly the crew were unable to send out an SOS. Given that the design and buoyancy of most modern ships is such that, in the event of an imminent capsize or other emergency, there should be sufficient time to deploy lifesaving equipment and fully evacuate, the accident had serious implications."