It is pretty well known that Google assigns a trust factor or trust rank to sites as a whole. I have personally seen it come and go with sites. What this means is that if you are a pure white hat with your site and you are doing everything perfect, you have a huge readership, nice level of inbound links and you write about any topic, in general you should rank well. But if you take that "white hat" site and start to push the envelop with some "grayer" tactics, such as some linking schemes or tricky on page SEO tactics, then you may see a drop in rankings.
A WebmasterWorld thread asks if trust rank can be taken away? The answer, in my opinion, is of course and it can happen all of a sudden. For example, you can have a great site with huge Google trust. Then implement something that may be frowned upon by Google. But it can take weeks and even months for Google to spot it with their algorithms. But when they do, you can find yourself ranking on the tenth page for a very specific keyword phrase that you used to rank number one for (and no one searches on that phrase also).
I have seen it, it happens.
WebmasterWorld, Senior member glengara said;
I'd suspect it's a combination of being a PR8 and the sudden massive increase in monetised pages, IMO you may have got away with it with a PR4 site, or if there was no monetization..
Now the "monetization" part of his line was hit big in the forum, where everyone argued with that. But 'getting away with it on a PR4 site" and not on a PR8 site, may have something to do with it. I do not know for sure, but I can see that come into play.
Forum discussion at WebmasterWorld.