I think perhaps I should expand on my previous comment that the root of atheism is reason.
I think that it is people's curiosity and the resulting thoughts of that curiosity. People just aren't content to accept answers like "just because Indra says so", or "just because Allh says so", or "just because "God" says so" and seek deeper explanations about the universe, its contents, human behavior, and life in general. We seek reasonable answers to our reasonable questions. It's easier to relate to "Joe Juniper lies about screwing around on his wife because he has antisocial personality disorder" rather than "because he's evil".