indyank - 5:50 am on Sep 19, 2011 (gmt 0)
It might even be that your site had just been pushed into the sandbox. When there isn't sufficient number of sites talking about a keyword and the search volume for that keyword is also low, google wouldn't obviously push the site into the sandbox. But now that more trusted sites are having stories for that keyword and the search volume is also probably increasing, google would have triggered that part of the algo that pushes newer sites into sandbox.
In fact, I'm convinced there are also parts of the algo that only operate above a search certain volume. So as long as a site is ranking well on a low volume search - it doesn't really get much scrutiny, either human or algorithmic.
This is exactly how their panda algo operates and as I explained in another thread, this is also the reason why sub domaining is helping Hubpages.I am also now seeing a similar sub domain strategy helping other content farms like suite101.com, articlebase.com etc. RIDICULOUS!
But the OP's situation is a different case and it might be the sandbox algo that is now working against his site. The sandbox algo places more weight on the number of available results for a keyword and then it probably uses the search volume to make its decision on a new site.