In the book, it is hinted that the Balrog of Moria is behind the bad weather on Caradhras.... imagine it.... at the end of the First Age it goes into hiding, or is trapped during the war of Powers.... the dwarves awaken it delving for mithril.... it realises that its old master, Morgoth, is long gone, and that Morgoth's old lieutenant, Sauron, is diminished... living close to Angmar and to Hollin it would not take long for it to discover the tale of the ring.... perhaps it too encountered Gollum during his wanderings in Moria.... then, joy of joys, the ring comes within reach.... with Morgoth out of the way, and Sauron weak, now is the time to size power, and become the new dark lord.... true, there are fellow Maiar walking the world, but these are weak, and should be no threat....
well, that's my theory...