http://allendowney.blogspot.com/2016/03/the-retreat-from-rel... This blogger has been covering the phenomenon among college freshmen since 2008. Notably the most recent results segregate none, atheist, and agnostic whereas before it was just 'none', but the effects are the same regardless of which label you want to associate with / avoid.

Personally I think the actual cultural movement towards 'atheism' is simple Leftism. Most people may still believe in the belief of God and thus differ from atheists on the ontological question, but their beliefs about the world in general are vastly different from those of their (for some religions, even still living) ancestors of the same declared denomination and those beliefs are in conflict with classic doctrine.