DU and other high profile sites use Cloudflare to protect against
denial of service attacks. It was checking if you are human.
It does that for me every once in a while. No cause for concern.

If I’m quick enough I hit reload and it reloads page without that popup reloading

No idea where it comes from

✌🏻

But a lot of websites ARE getting fairly persnickety about browsers and settings, etc. I've noticed problems where there were never any before ...

Depends how much I am on du.