tedster - 3:29 pm on Dec 8, 2012 (gmt 0)
there were no changes made to the overall site and it's not because I have some hot new content on the index page
Right - I see the same thing. It definitely relates to a change that Google makes, and not one that the site owner makes. Knowing whether to send traffic to a page that is higher up the pyramid or down in the more detailed area seems to be something that changes according to metrics that they are watching, and the changes can happen in an automated fashion without requiring an "update".
I have found that eventually these changes settle down. Only if it settles down on what I feel is the poorer choice of landing page do I start changing things - and the first place I look is keywords in internal anchor text - especially for content area links, not navigtional links.