Recently a lot of webmasters in WMW forums reported a significant loss of traffic from google, Also they faced major drop in keyword rankings, Here is a brief post by a guy who started that thread,

Possible reasons speculated by the poster:

1. Are they affiliate sites with many links to the affiliate host.
2. Do they have satelite domains which they cross link/one way link with sites particularly on the same ip c-block.
3. Are the drop in serps keyword specific or site wide.
4. Are some inbound links on mass from single domains. For example, a great number links from a footer link on a major site.
5. Have changes been made to keyword density recently.
6. Is there a distinction in ranking between major and minor terms.
7. Has there been any PR changes or backward link changes.

Read a long 5 page thread here (webmasterworld.com/forum3/25465.htm ) if you are interested,