Evolution and Atheism are not associated. Plenty of religious people accept evolution. There are no beliefs associated to atheism other than a lack of belief in any diety (and thus religion).
As far as why some atheists are openly critical of religion, there's a variety of reasons.
First of all, of course there's jerks in every pool of people, Christian, Islamic, religious or not. That probably counts for most of the annoying people.
Though really, its hard to resist arguing (or defending yourself) when now and throughout history, pro-religion or pro-Christian groups openly imposed their beliefs on others, denigrated 'non-believers', and assaulted atheists as communists, immoral, un-American, and so forth. In some areas, openly being atheist can still land you ostracization or even subtle harassment from those around you.
And truth be told, not being believers in religion, some atheists may see things like evolution as a simple matter of rational analysis and discussion, not religion, so will argue openly against the dread foe 'ignorance', not really understanding the significance of the religious implications to the other.
Myself, I have some quite religious acquaintances, and even though they don't intend on doing it, inevitably they end up putting a bit of pressure on me. Simple things, like implicitly asking why I believe what I do, why I don't believe what they do. Either I have to hold myself back and basically dodge the question, or I have to stand firm and assertively state my case. And in any case, it also tends to identify me as an outsider.
Anyway, being in this kind of environment, it's not surprising that atheists may feel like they have to fight for their right to exist. At my uni, an atheists and agnostics club formed in part because it can often seem pretty difficult or lonesome out there when you're surrounded by people who are 'believers'.
While it may be hard to fully realize, atheists are effectively a religious minority, and often have to deal with the prejudices and generalizations that the 'majority' tends to make against them. While it depends a lot on the region, in general some nominal belief is expected. Being a rarely practicing Christian is no big deal, but atheism can make people look at you differently. It's hard to quantify, but a bias definitely exists. After all, a poll once concluded that atheism is one of the biggest traits that would cause a person not to vote for a presidential candidate. A classmate of mine once lead the 'Pledge of Allegiance' in high school over the P.A., and he omitted 'under God.' For that, he was forbidden from leading it again by the administration and chastised.
So anyway, I apologize for kind of going on... I guess I wanted it off my chest. Atheists typically don't act the way they do for no reason, at least not the ones I know.
~k