fred9989 - 10:09 am on Apr 26, 2012 (gmt 0)
Also would add that I supect Domain ownership as identified by WHOis (rather than linked websites) may be at play.
I say this only based on the interesting (well, to me, anyway) history of one of my websites.
I wrote a book years ago (subject:a very popular hobby) and when the book went out of print, put it up as a website.
Left unregarded by me, unknown to me it attracted hundreds of organic links through being spotted and promoted in the forums, in fact it became the authority in its field, and ranked #1 for a popular term for years.
It too has now plummted, and one possible key might be either the WHOis or the fact it is on the same private server as all my other websites with a hosting company.
But on the face of it - there is no other reason for the drop - no spam, no SEO, nothing artificial whatsover, and it has great content (if I say so myself, LOL).