Why are atheists aggressive?

I'm not talking about the uneducated atheist trolls that go around the web saying "f*ck God!". I'm referring to the formal educated, well-mannered, and somewhat joyful atheists. I've watched videos of Richard Dawkins and Ricky Gervais and I think that atheists are just a group of misunderstood people. But now I go to a book store and I see a whole shelf packed with atheism books. So why take action now?