Wilburforce - 4:37 pm on Aug 20, 2013 (gmt 0)
My best guess is that this is happened because (in Google's eyes) I basically abandoned the sites.
Earlier in the year the site of a business in my niche that ceased trading in 2009 floated up to page 2, remained thereabouts for several weeks, then slowly sank again (although it is still head of my own site for a couple of terms). I'm not clear about why the website is still live, but it hadn't been changed at all since the business failed.
Clearly abandoning a site doesn't always mean it is lost forever to Google.