Atheism itself, atheism as such, isn't and can't be a movement, because atheism is, at a minimum, simply non-theism: non-belief in any god. Mere non-belief in any X can't by itself constitute a movement, because it's merely an absence (or at most a refusal) of belief. If every absence of belief in [your chosen belief-object here] amounted to a movement, the traffic jam would be a nightmare. A belief about the world shouldn't necessarily commit us to political action – we have to be able to say "No" to affirmative beliefs about the world without thereby signing up to a campaign. We need to be able to make such choices more freely than such a requirement would allow.
Atheism can however include something like a movement, of course, as can other beliefs and non-beliefs. Some of the disagreement among atheists is around this issue. Many atheists want to be able to be atheists without being dragooned into some boring noisy unsubtle