Something like this was bound to happen. I knew it, and I think you knew it. We just didn’t know we knew it. Matt Cutts posted a video the other day explaining why there will be some older sites that won’t always be able to keep their rankings on the Google search engine forever.
Having a site that has been around for quite a while (in the case of many years), can be a good thing. It can created tenure and will undoubtedly be entrenched in the search engines. You can call it tenure for websites. But, there are some sites that are about 15 years old, and because nothing has really been done with them when it comes to their original design templates, not much has been done to improve them since the sites were created.
To be more specific, there has been no updates to user interface, redesigns, and no new features have been added to improve the experience for the users. The lesson is to stay new, stay fresh and remain competitive. Take a fresh look at your site and make it happen.