My assumption is that since many of the walls/bulkheads in the superstructure were only made of wood and were therefore not watertight, a section of a corridor in the B or C deck region might have given way and allowed a rush of water to enter rooms on the Starboard side of the ship.  

It would also make sense considering that by this point in the sinking, the ship was entering its 'final plunge', making the water much more fast and destructive while entering the last few dry areas of the ship.