As of the last crawl of your website, you appear to be running vBulletin 3. One or more of the URLs found were:
Google recommends that you update to the latest release. Older or unpatched software may be vulnerable to hacking or malware that can hurt your users. To download the latest release, visit the vBulletin download page. If you have already updated to the latest version of vBulletin, please disregard this message.
If you have any additional questions about why you are receiving this message, Google has provided more background information in a blog post about this subject.
Funnily enough, I haven't been running vBulletin for nearly a year (sign that Google doesn't understand my site?). Not only that, but the link to the blog post about the subject indicates that the feature has existed since 2009. So I never got the notice for the years I was running vBulletin, but 9 months after I no longer am, I get the message??