Ships that end up upside down on the bottom may have capsized before sinking, or while sinking. Either way, the effects on the rest of the ship are the same. In the 20th Century, warship turrets were held in purely by gravity. Fixing the turrets in added an extra point of failure, and was unnecessary - after all, a ship in perfect working order would never roll over far enough for the turrets to fall out. As such, when a ship capsized, the turrets slid out, and sank independently. When a ship sank on an even keel, the turrets stayed with the ship. Similarly, superstructures (which are built quite lightly, compared to the rest of the ship), are designed to stand up when the ship is the right way up. When a ship capsized, its superstructure (which has likely been weakened by shellfire), will experience loads which it was not designed to support, and may collapse. If the ship remains capsized all the way to the bottom, then the superstructure will be crushed by the weight of the hull.