In response to the Quora question “Why do some women insist they are not feminists despite clearly believing in the tenets of feminism?”, Yishan Wong replies:
Feminists often have the same problem as libertarians, which is that their group is sometimes viewed as obnoxious or overly combative by others. Thus, individuals who agree with tenets of the philosophy often wish to disassociate themselves from the movement by disavowing social identity despite expressing philosophical alignment.
The core problem here is that philosophical alignment is often conflated with social alignment, especially by unsubtle individuals (the ones most prone to giving people grief socially); as a result, a non-trivial number of adherents to either philosophy will seek to distance themselves from the “label” in order to avoid the trouble of social friction.
I claim that the same is true for atheists as well.