90% sure as i Have no ther links on the site, I removed the link put in a reconsideration request and got the standard message and the first message disapeerd.
Its the only thing i can think of unless it coincides with another update but been steady number 1 for 4/5 years
The way i see it i had a PR5 car site pointing at a PR 2 site, and although yes i owned both sites they are not really niche related, and the message i got was
We've detected that some of your site's pages may be using techniques that are outside google's Webmaster Guidelines. Specifically, look for possibly artificial or unnatural links pointing to your site that could be intended to manipulate pagerank. Examples of unnatural linking could include buying links to pass pagerank or participating in link schemes. We encourage you to make changes to your site so that it meets our quality guidelines. Once you've made these changes, please submit your site for reconsideration in google's search results. If you find unnatural links to your site that you are unable to control or remove, please provide the details in your reconsideration request. If you have any questions about how to resolve this issue, please see our Webmaster Help Forum for support. Sincerely, google Search Quality Team
So it may be a coincidence but it makes sence point a higher pr site to a lower one could result in a message like that, however that message was in my WMT on the 16th december the drop took place on the 16th of January