Think of it being like your anti-virus software on steroids (they do a PC version too) that updates itself every day (sometimes more than once a day).
What generally happens with groups of servers is that there is a "staging" server that downloads the patch from the internet, then delivers it securely to servers. So they don't actually need to be internet connected (which is of course really dangerous). If you're lucky, by the time you spot and issue, you can shut down the staging server without it doing too much damage.
Now thinks of this anti-virus software deciding that part of Windows is actually a virus, so it kills that process. Except that the process is key to letting the machine work. So it crashes and shuts down.
That's basically what happened here.
The problem is, these servers individually need to be brought up into safe mode and manually fixed.
To put into perspective, if this happened to us, we'd be talking about 40,000 servers that we're responsible for alone.