I know it might happen to everyone.. actually there are lots of factors where investigating why Panda throw our site in Page 8.
Factors to consider:
1) private blog networks (blogroll/blogpost)
We often used services from the bhw. Last updates, most of the service providers were open to us about the deindexing of some of their sites (which is really good to the customer because they are really honest of what's really happening, we greatly appreciate that). But I just figure out that, its not just 1 campaign that they are working on, there are actually bunch of campaigns, but the question here is that why is that only 1 campaign lost its rankings.
Hypothesis: Maybe Big G didn't actually penalize the sites that have a link to the private blog networks that have been deindex.
2) Mass Same Deep URL link
Recently, we purchased a blog post for 8 campaigns. The 5 campaigns have 3 keywords and pointing all to homepage of the sites. The other 2 campaigns have 3 keywords and pointing to 3 deep links (/about | /blog | /faq ) . The last campaign (i'm talking to the campaign that have been thrown by panda to page 8, have 3 keywords but mistakenly link all the 3 keywords to /about page. (all campaigns have 100+ backlinks in just 16 days)
Hypothesis: Maybe Big G see this campaign as unnatural building of links since in just 16 days, it has over 100+ backlinks all pointing to about page. But I figure out that this might not be a factor since the links still haven't been change but we already claimed again the throne for all our primary keywords (most our the most competitive keywords).
3) ON PAGE SEO
Last October 2011 (If i'm not mistaken), we did a redirection from a site to another site, except its not successful because the /blog haven't been redirect. We tried to fix this just this week, and when I checked the site this morning (we're now #1 AGAIN)
Hypothesis: I'm not sure since the redirection happened really long ago, but we lost the ranking just this Feb.9, and after fixing the redirection, we're back again kicking.
As one bhw member post a reply from one of my thread recently,
"GOOGLE IS LIKE X FILES, YOU'LL NEVER KNOW WTF HAPPENED"
Thank you