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, 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.
The impulse engines were also online. Even if they were dying, just that little extra push would have been enough to tip her over the the edge.