The power to reject the President's Cabinet nominees is a direct Constitutional power of the U.S. Senate, but it is rarely used. The President, even the most zealous Senate partisans normally feel, gets to pick who he wants to be on his team. But a few times in history, that theory has been discarded. And when the Senate does unsheath it's constitutional power, it means that things have gotten quite ugly between the two branches of government. Perhaps it means the President has overreached. The party controlling the Senate is showing off. Or, there could be no reason at all.