It appears to me that they are going after the spammy ad sites as well as many MFA sites that have nothing above the fold except a few large blocks of ads. You have to scroll down to see the content.
The majority of these site that I have run into are using AdSense. They do not comply with the heat map layout because there is no header. The only thing that you see that is above the fold is ads.
It is also interesting to note that Google is now officially calling this a penalty, after denials that penalties existed for the past few years.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin