As you already know the ship could only stay a float with the first four compartments flooded. The water- tight bulkheads only went as high as E deck so once the front of the ship went under, The water simply spilled over the bulkheads filling each compartment.
This made the aft of the ship extrmeley heavy and simply pulled the ships stern up into the air. The ship was not desighned to take anywere near that much strain, The bulkheads, supports and the hull just buckled.

I do not beleive it broke up below the surface. The stern at this point would have been full of trapped air and would support itself on the way down.

I aslo do not think hydrodynamic forces were responsible for this either. The vortcies created by the ship on the way down however, would have resulted in the ship's super structure dicintergrating.(i hatsen to add that vorticies or turbulent fluid flow increases as the speed of the ship increases, therfore the most damge done by turbulent fluid flow(vorticies) would have been done seconds before the ship hit the bottom. So turbulent fluid flow can not be responsible for this.

I therfore conclude that, I beleive that the emense strain put on the bukheads, hull, interior structural support and superstructure
ultymatley ended in the ship buckling under it's own weight and breaking up on the surface.

thanks for your time.
Shaun.