Ulfric,and the storm-cloaks in general are not only helped alduin arise,but also helped the thalmor. The thalmor are more obvious.With the empire divided,they have neither the time or resources to drive out the thalmor,and that's just the way the aldmeri like it. If the storm-cloaks destroy the empire,the aldmeri would wipe them out without a thought.The entire empire couldn't defeat the thalmor,I doubt the storm-cloaks can.
Alduin is more of a theory of mine,but a very solid one.A quote from the loading screen is And the scrolls have foretold of black wings in the cold that when brothers wage war come unfurled.Alduin,bane of kings,ancient shadow unbound,with a hunger to swallow the world. Alduin is the Nordic god of destruction and chaos.Therefore,when brother and brother fight,when grief and sorrow are rampant across skyrim,would create enough chaotic energy for him to arise.Also,the fact that alduin conveniently keeps ulfric from being executed can't be looked over.
Another thing.I haven't finished the main quest-line,so no spoilers!