Originally Posted by
Vyrerus
I believe it's stated at some point that Great Wyrms can still perceive events happening around their eyes, as long as they are alive... which is also directly tied to their eyes on some level.
I've always inferred that the answer to this question is that Nidhogg basically saw Thordan VII making his plans to become God King Thordan, and so he sped up the pace of the war to try and stop that.
As for why he didn't come himself to try and batter down Ishgard's door/do the deed himself... well, as the fight in The Aery with him proves, even just having one of his eyes allows someone to enfeeble his already enfeebled form, so it wasn't an option.