CainIV - 6:18 am on Jan 26, 2012 (gmt 0)
By any chance does your subpage generate more usage signals than your homepage?
Be careful when internally targeting pages to the same theme as the index page theme set and especially so when building links. Some call it keywrod cannibalization.
I see Google switching off internal pages with homepages in this same fashion from time to time, often replacing the exact position of the homepage listing with the internal page - almost as if some type of internal usability test is being done to select the better of the two pages.