I still don't get how Ishgardian astrology was supposed to work

[Jandor]

Each planet has its own life-stream, maybe the dragons, especially Middy and the first brood that originally came from there, retain some link to their original world through that.

When the Dragonstar aligns with X, Y & Z it's aetherial fields will be particularly swole, and by extension so will Nidhogg's own aether, an attack is likely at that point. Something like that.