Unreasonable Faith floats a theory:
Atheists have always been a minority. Religious minorities are frequently in an awkward position, particularly when the majority considers their very existence to be a challenge. So atheists have tended to keep quiet, sometime not even realizing that the person they are speaking to is another atheist.
The internet has alleviated some of this problem. First it provides a semi-anonymity, which allows people to speak freely. Second, it’s created a way for people who are geographically spread around the world to meet together and discuss. So the internet provides something of a support group, which makes the atheists stronger and more confident. This also produces a group polarization effect, which makes the stronger atheists more confrontational.
So when folks like Dawkins came along, there was a ready made audience for their work. The success of The God Delusion helped get other atheist works published, creating the wave of “New Atheists” we see today.