Some of that movement came after Meygan Kelly campaigned for Rubio in the last debate.
FR THREAD: No, Ted Cruz Did NOT Support Amnesty, Concedes Megyn Kelly AFTER #GOPDebate
That may be, but it still stands that Rubio did much better than expected. That creates interests in him in the next election. He did well enough to pull ahead, as he certainly didn’t do so poorly as to be discounted out of the race.