Since the early days, the AdSense team has been pressing publishers to increase the aggressiveness of their ad-layouts. It started with the AdSense heatmap
[google.com]. In the graphic on that page, content clearly takes a back-seat to ads.
Then in the aftermath of the Panda update, Matt Cutts said the following in a wired.com interview
Does this site have excessive ads?
This put the AdSense team and the Search team at odds with each other: AdSense was pushing aggressive layouts, and the Search team using aggressive layouts as a negative signal in their algorithm.
Today, AdSense released a best practices guide
[google.com] that seems to try to balance increased monetization with improved user-experience.