I've noticed the basic fact of life that denying things is inherently more distressful to people than asserting things.
I could talk to people and assert crystal power, homeopathy, and ancient aliens all I want, and people will still perceive me as a positive, happy human being (even if they think I'm wrong and crazy).
But denying what other people assert is ALWAYS a dirty business, because you're essentially denying not only the belief, but the believers. There's always the issue of being perceieved as "attacking others" when denying certain beliefs, and that draws some measure of negative stink or outright hatred when in the process of denying.
The thing with atheism is that you only will see the tip of the iceburg of all people who are truly "lacking theism", because of this imbalance between asserting/denying. Some atheists will keep their mouth shut, to avoid "disrespect". Some don't care who they offend, and will broadcast it without a care. The nature of atheism fosters iconoclasts as their strongest voice, because they don't care who they offend. And some people who generally hold respect will be seen as "disrespectful" anyway, simply by means of asserting their position. The result is a heavily skewed idea of atheists as only the worst kind of intellectual rabble-rousers. The broadcasting of the position favours the most brash individuals, and so it appears that the belief is brash. But it is not so...
Long story short: the very position of atheism tends to foster the idea that atheists are annoying, society-disrupting haters, when it's really a problem of denying other's beliefs which creates a false stereotype. The average atheist may not resemble anyone's stereotype of an athiest.