We don't actually serve any captchas: it's all done by Cloudflare. Cloudflare runs a firewall that scans incoming requests and issues a captcha challenge to any that appear to be automated or malicious. The exact rules it uses to do this aren't public, but we can tweak it some. It seems to trigger more often (though not always by any means) if you're posting code, particularly PHP. If you let me know the details of any request you've made that got hit with a captcha, I can have a look at the settings and try to identify any issues.