@adamgottlieb Typically it isn’t just because one page has a lot of links when it comes to Google penalization.  If so, my blog would have plummeted a long time ago because I link to a minimum of 25 external sites every Friday.  That and I have created posts with 50+ resource links in them for others, and they haven’t had issues.

What I would suggest is taking a look at when your site got hit, usually detectable in Google Analytics if you look at your search traffic referrals, and match it with one of the Google Algorithm Change dates listed here: http://www.seomoz.org/google-algorithm-change.

If you can pinpoint it to a specific update, read the related link about what Google has targeted and see if you can find something on the list matches something you do on the site.

From my own five second look over on your website, I would guess that the reason is the links in your sidebar under your sponsors – Google has been making a big crackdown on selling links and placing them on your website without the rel=”nofollow” attribute.  Of course, most sponsors who buy text link ads won’t buy them if you do mark them as nofollow – it’s a catch 22.

@adamgottlieb I think Google finds them based on who they are penalizing for buying links.  For example, if they found out Site A was buying links based on the fact that they have tons of sitewide links with the same anchor text on lots of different sites, they can ban Site A plus assume that any site with a link to Site A sold the link, therefore go after those as well.  It’s more based on an algorithm.  

I know that in the past when I worked with link building companies, we would target the same sites that we knew we could buy links from, so several of our clients would have purchased links on the same sites.  Hence if Google figured out who our clients were, they could find out who was selling links to us too.