It can't be Sauron, because Sauron would at least be able to sense that Gandalf was there (remember when Gandalf said that when he used his staff to light the fire), and even if he couldn't the following still applies. Sauron would not have stopped the storm after they decided to leave the mt. He would have continued. And if Sauron could control the mt. why did he not control so many other things? So I think it was not Sauron, and this sort of leads me away from thinking it's an entirely evil thing. It could be just some disgruntled power that does not want to be bothered.