There seems to exist a sub-set of the general atheist population who's main reason for labelling themselves "atheist" is in active rebellion against "christians", rather than because they knowingly lack a belief in gods in general. It's self-definition through hatred of another group. Very negative, and, in my observed opinion, just another expression of that dark side of humanity we usually see as racism, or sport (soccer) hooliganism.
Fundamentalists are frequently easy targets, and I have been guilty myself of shooting those particular fish, but I do it in their barrel (if you'll forgive the distortion of the metaphor), so they have the option of shooting back.
Having a group such as "Thinking Atheists" (on Facebook) is, IMO, cowardly. The group posture together, bragging about their superiority over those terrible fundies, hooting at the perceived inadequacies of the fundies, but without the courage to face them on their own ground. Like the media image of redneck kids in pick-ups hunting lone coloured folk for a beating. As soon as they feel threatened by an outsider, they gang up on them to drive them off and bolster their fragile self-image.
Basically, it's a part of humanity that needs to boost itself by discriminating against others, and is running out of "others" they can get away with discriminating against.
Speaking as somebody who could legitimately carry the title Thinking Atheist, I could do without "their sort" using my world-view as an excuse to beat up on a vulnerable minority.