First of all the devs successes said we will not be leaving the Shadowlands for the expansion. They already kinda broke that too, because I had at least 2 quests for the Nightfae that let me to Azeroth (Twilight Highlands and Nazmir).
More inportant questions:
1. Why invade Azeroth? Why not one of the countless other worlds? Is it that world soul thing again? Might be.
2. Why shatter the veil when he was CLEARLY not ready to invade? Currently it seems like he is not even close to invade.