That was one of the things I mentioned. It's not inherent to fundamentalism, but "nu-uh, you're wrong" goes hand-in-hand with fundamentalism.
An established doctrine is not necessary; adherence is, and many atheists do place a fundamentalist sort of faith in their beliefs. For example, some atheists resort to current scientific understandings as the building blocks of their beliefs, even though the very nature and essence of science is change and discovery, which renders all atheistic views established purely upon science illogical, as it is a given that what we think we know today can be changed tomorrow given a new discovery.