Here is my theory.

Google instant does an even better job of getting the user to use their predetermined search query rather than continuing to type out the users longer tail search query.

By funneling search traffic into these head terms, Google is concentrating traffic around the terms with higher AdWords CPCs. And as more and more users are funneled into these terms it will increase the level of competition bidding on these terms, further driving up the CPC to appear of the first page.

Seems the most plausible to me!