I think this is probably because the perception of self-identified atheism among people younger than twenty-five or so is no longer dominated by people like Dawkins, Harris, etc. That and the fact that it's especially clear in very-young online spaces that "nothing in particular" says nothing about one's actual views and that plenty of people who describe themselves that way are at least as nonrational in their approach to fundamental questions as most religious people.
Worth noting that, according to Pew, only 13.5% of "Nones" are atheists, and only 17.5% are agnostics. A full 61% say they believe in God, with 49% being "fairly certain" or "absolutely certain." Atheism is growing, but it still represents a very small minority among Americans who are leaving organized religion, and that is certainly not the popular narrative, from my experience.
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