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That can happen if your browser requests are malformed, or you are using an IP address with a poor reputation (i.e. something like Tor, which gets used by spammers and those attacking other sites).
Basically this site, and about 10% of the internet, uses CloudFlare to stop the site being compromised or taken offline. And the basis of that is this idea of IP reputation and good/bad browser requests. If you look kinda odd to that analytical layer, then you're given a challenge to confirm that you are a person and not some nasty bot trying to steal data, or compomise the site.
I've just had Cloudflare interrupt proceedings with a "checking your browser before proceeding. DDOS protection by Cloudflare" screen. What is this shit?