The events of 9/11 forced Ayaan Hirsi Ali to examine her beliefs; in May 2002 she realized that she was atheist (Infidel, chap. 14, Leaving God) and that in practice she had left God years ago. If one assumes, with Hirsi Ali, that the goals of religion are "to be a better and more generous person", then the motivation toward atheism comes from the realization that the methods of religion are not suited to promoting this worthwhile goal.
There has been some disagreement on the origins of what has come to be called New Atheism, the greater visibility of atheism in popular culture and the media with notable authors such as Dawkins and Hitchens, that began around 2004. Richard Norman is quoted by Caspar Melville, of the New Humanist, as saying that the impulse began with 9/11. Melville's article, which seems to have arisen out of Melville's own inability to discern any positive direction from new atheism, has been criticized by several bloggers. This point in particular was dissected by Aratina. After reading Hirsi Ali, I'd like to weigh in as well with an entry of my own.
Did it begin with 9/11? Undoubtably 9/11 had a very polarizing effect. For some authors such as Hirsi Ali and Sam Harris, it was a clarion call to defend reason and a confirmation of their own stance on religion (though many other authors needed no such confirmation at all). The question can put another way: Without 9/11, would new atheism exist? I think that it would, because 9/11 is not an isolated example of the negative impact religion has had on society. If one were to cull all references to 9/11 from the books and other works of the new atheists and their supporters, you would only have removed a very small fraction of the material. Unfortunately, the grievances against religion are far more numerous than a single event in 2001. (Is it worth mentioning that the destruction of the twin towers of the world trade center in New York was foreshadowed by the destruction of the twin Buddha statues at Bamiyan?) Additionally, each of the authors built upon the success and awareness raised by the others. As the books came out in relatively quick succession they were able to ride the same wave, pushed a little higher by the publication of each book.
I would miss an important aspect of new atheism if I didn't mention that it is defined not just by the popularity of the recent authors, but notably that a significant portion of these authors place emphasis on a scientific perspective, where the question of God is treated as a testable hypothesis (that fails), and demonstrating the importance of evolutionary thought in better at explaining the origin and nature of humans. In this regard, Norman is right in that it does appear to address Christian fundamentalism, which denies science preferring instead a literal interpretation of the Bible. The first Age of Enlightenment was built upon the realization that evidence and the methods of science could lead to a surer foundation for knowledge and progress, so it is encouraging to see these tools employed by the new atheists in the service of reason again. Another point is that new atheism wouldn't even exist as we know it without the online community of nonbelievers that has come together and flourished as organically as the Internet itself (see Aratina's post for a more full description of this aspect).
As I mentioned much earlier, I denied my religious belief in May of 2001. For myself and many former believers, it was a significant decision not made lightly, and I recorded the date and exact reasoning that led to it. This was obviously four months before September 2001, so I had no knowledge of what would happen later that year. It is interesting to note for myself, that my infidelity with religion predates Hirsi Ali's, which became famous several years ago after the publication of her book, Infidel. I do not know anyone whose views became atheist after 9/11 that wouldn't have come to this conclusion of their own accord without any additional impulse from that tragic event. Incidentally, reading Hirsi Ali has been a pleasure, she is capable of conveying a wide range of emotion- an epilogue to her most recent book Nomad is titled “Letter to my unborn daughter” and it is very moving.