My opinion is that it snapped back on the bow's impact with the bottom. Chances are that if the mast had snapped back on descent, the backrush of water through which the ship had knifed would have probably washed the mast away, or the movement of the falling bow would have caused the mast to roll off the side. The descent may have been straight (or, as I believe, arced), but everything not secured to the hull or decks would have easily been blown away by the force. The speed of descent would be consistent with this probability, as it took a mere ten minutes for the bow to strike the ocean floor. As we see on the wreck, the base of the mast is lying right next to the spot at which it was once erected from the forecastle deck. If the mast had broken free near the surface or on its way to the bottom, the chances are unlikely that it would have remained close to the point of the ship from which it had broken free.
But, as said: That's just my opinion...