I searched https://cs.stackexchange.com/ for "Halting problem" and got redirected to a captcha. I'm not even modifying anything, just searching. What is prompting this?
All anonymous use of the search engine requires passing a captcha.
If you log in to the site the captcha disappears.
Searching takes resources too, and apparently some robots have been trying to use the search engine a lot lately. The captcha is there to limit that abuse.