Interesting note about Enterprise falling to earth:
Right before, Spock warns the crew to brace for a shock wave from the exploding torpedoes he beamed over to Vengeance.
Could it be, perhaps, it was that shock wave that pushed Enterprise into earth's gravity well causing her into free fall?
Also, did Vengeance crash, or did Khan directly aim Vengeance into earth's atmosphere? It wasn't careening out of control when we see it fly past Enterprise on it's way toward SF. It was taking a straight shot down.
Right. Why hasn't anyone thought of that before?
They have, and that if you watch, the Vengeances last few phaser and torpedo impacts are hitting one side of the ship and shoving her in that direction, the Enterprise was under the influence of several kinetic forces, none of which she had the power to stop.