We had a large number of DDoS (Distributed Denial of Service) attacks across our web servers, most of them directed at the administrative areas of some software packages, including Joomla! and WordPress. These attacks flooded these sites with requests and eventually caused our servers to overload.
We have now installed a security feature where if a single IP address has logged a large number of requests to the administrative login page, it will produce a CAPTCHA system for that page.
CAPTCHA is regularly used across the Internet to block access to bots, as they are unable to read the image code and enter it in to get past the security check. This keeps your site safe and also prevents our servers from getting overloaded.