In a bitter twist of irony, the motivations that drive the most vocal atheists are precisely those that drive the most vocal religious zealots: "I'm right, why doesn't everybody see that?" There's a big slice of humanity that doesn't feel comfortable about their beliefs unless they're being reassured, whether by Winning in an argument or by mutual congratulations from like-minded thinkers. And it spills over into their other interactions with people. Given that I'm going out of my way to make this post, I'm probably one of 'em. Trust me, though, it's nothing special about atheism; or if it is, it's that it's in a cultural context where the default is religious, so that in order to be an atheist you have to be willing to stand out as part of your identity, so there's some self-selection for people who are happy to fight about their ideas in general. That, and in a lot of places, there's a fair bit of prejudice against you that makes that sort of lashing out a defense mechanism.
Not sure if an atheist, though. We've had big fights on these boards in ages past about whether or not agnosticism falls under that. Although, what I believe gets increasingly difficult to explain, of late, the more it becomes internalized and interpreted in terms of my own particular experience. Generally happy to see people do what the fuck ever, though. I'm happier faced with mysteries than not.