Google thinks I'm not human

While surfing from the base2 project-page to our Google Group, I got an error message, stating I might not be human. After filling in the Captcha, I could proceed.

I’m wondering: based on what information could they suspect my request was automated? Maybe it’s because I’m behind a corporate proxy. Funny. If this becomes popular, creating “webpageservices” (scraping html pages) will become more problematic.