In a way, yes, you could say that Ulfric caused Alduin's return and you would be right.
You can also say that the Dragonborn mage with the Elder Scroll caused the Dragon's Return, and be right.
You could blame the very existence of the Elder Scrolls and be right.
You can blame Akatosh for creating Alduin and be right.
Hell, you can blame that random mudcrab you Fos Ro Dah'd for nipping you in the bum, and you would still be right.
THE RETURN OF ALDUIN WAS GOING TO HAPPEN! Everything that happens in TES games is part of some master universal plan, and everyone has their part to play, so pointing fingers at specifics is pointless.
...although i must admit that mudcrab is pretty suspicious...
