If they posted it on an error or outage page then they probably didn't mean to set it that way, and that implies that there was a non-obvious mistake. They might be doing something silly with their permissions.
And that is presuming that this is some sort of technical issue.
"As part of an internal change task" is the justification listed. Maybe this is a genuine accident.
Someone paranoid might think that the for-profit management at Elastic is trying to pull some of their previously free software behind a paid-for product. Perhaps they accidentally marked all repos private when they only intended to make a few of them private. They have had beef with AWS in the past where they changed their licensing due to things AWS was doing. So I'll fully believe that it was a genuine accident if all the formerly public repos become public again.
It's a configuration error (sorry!). Also with thousands of forks this would be a pretty pointless operation. Once something is out (and that includes a license), you cannot just take it back — it will be there forever.
I seem to remember someone posting about this once -- you lose all your stars / followers when going public -> private, and they're not restored when you go back.
I would bet, as a result of this and other things like fork management, that they'll be working with GitHub support to try to reverse the go-private and all its consequences.
If it's this: https://news.ycombinator.com/item?id=41060102
Then they will need to delete(or rename), remake the repos and push again. Any security problem would also require doing some due diligence to make sure you really squashed it.
Yeah. This was a configuration error. Keys you just rotate. Making repos private accidentally creates a whole new mess with forks, stars,... Not recommended
And that is presuming that this is some sort of technical issue.