It was the fault of the Keel. The keel was Titanics spine if you will, then there where struts going up the the steel over that. Very much like how a human body is designed. The Titanics hull was not designed to allow half the ship to be in the air so after the stell gave way the ship split into 2. But the keel which is super strong though damaged is still holding on so now as the bow went down the keel brought the stern up as at this point it was still attached. When the stern was up in the air the keel could no longer take the weight of the bow and si seperated. Basically the keel kept Titanic up as long as possible. If it was purley up to the stell the ship would of bent. As the stern was upright the water finally took over and the bouency was lost and it sank. The bow section goes down and crashes into the floor. The reason the bow is still recogniseiblr due to the fact it was full of water other wise the water pressure would of crushed it.

This would be plosible if you believe the film version. But I think it sepereted under the water and the stern twisted which would explain why the stern section is more mangled up.