I would guess that Blizzard is doing hardware maintenance to prepare for the realm connections starting later this week.
My other guess is that Patch 8.3.7, Which is on the Public Test Realms right now, Will be coming out to the Live Realms.
Note: Patch 8.3.7 is not a content patch. It’s a pre-patch for Shadowlands that is adding some upgrades to the way NPCs look in-game.
You can read more about Patch 8.3.7 by Clicking Here!