Not at all sure about this, but could it be people using the newly released SSL (https) version of Google? See this in their FAQ:
As another layer of privacy, SSL search turns off a browser's referrers New window icon. Web browsers typically turn off referrers when going from HTTPS to HTTP mode to provide extra privacy. By clicking on a search result that takes you to an HTTP site, you could disable any customizations that the website provides based on the referrer information.