I'll try answer this question as best as I can based on what is known.
The fact that Titanic didn't capsize is a mystery that is hard to determine why. I have done some brief research and it has mostly been linked towards the suitable design of the ship. It is known for a fact that there was a list to port during the sinking, however the ship didn't capsize, even after the breakup. Again, this is mostly summed down to the design of the ship.
Site member Dave Gittins responded to a question like that last year (which I have linked) by stating "
Quite simply, Titanic didn't capsize because she was pretty well designed. Water entering the starboard side could easily flow across the ship and keep her reasonably upright. There were a few obstructions, like the firemen's tunnel, but overall the design was good."
Now, once the ship slipped beneath the surface, the bow mostly descended calmly, albeit with damage occurring as the funnels were snapping off of their wiring, destroying the bulk of the davits. The mast also snapped back, destroying the wheelhouse, but other than that the bow descended pretty calmly until it impacted on the bottom.
The stern on the other hand, experienced an implosion within the first 200-400 feet under the surface. This caused the stern to begin spiraling out of control right until it settled as it landed on the ocean floor.
I'll link this here.
Answer by Gittins
Feel free to respond if you want anything clarified.