I too encounter this phenomenon periodically. I have noticed that it happens when using certain servers on my VPN. I can't explain the reason for this behavior of these servers, maybe CloudFlare considers them suspicious or something else, but when I switch to another server this problem disappears.
I face this whenever I am trying to access the forum using TOR. So I also think it's a cloudflare related thing. More like a security checkup. In normal cases without using any third party VPN or TOR like things it never shows up. Sometimes it would show up when the Internet connection is so slow. Apart from these cases, never encountered this.