First, you have every client retrying connections, whether automated or manual retries. This generates extra load over normal.

In addition, whichever systems come online first often get a ton of traffic sent to them, which can immediately bring them back down if you don't have ways to prevent it. You will get a wave of systems coming back online, a ton of traffic hitting them, and then falling over again.

Now, I am sure Microsoft is used to this and has procedures and systems in place to mitigate the issues (I know we did).

However, sometimes if the right combination of services are failing, even the strategies you had to avoid this sort of failure can themselves fail.

This seems to be the smoking gnu, now what would it be ...

EDIT: Seems like now they confirmed that: "We've identified that affected servers are failing automated health checks, which is causing them to become unavailable, and this may be related to a recent change to a component involved in processing those checks. We're testing a configuration change on a subset of affected infrastructure to address this and, if successful, will apply it more broadly. In parallel, we've initiated a restart of all affected servers and are validating whether they're successfully returning to service."