The bottom line is that the word 'atheist' has a lot of baggage (deserved or not), and people tend to pigeonhole you if you say that's what you are. That's why I prefer to say "I'm not superstitious" in such situations. Less baggage, and it's almost as good at getting a rise out of people ;-)
There are many that really seem to enjoy being atheist for the sole reason of pissing off religious people. Like I'm talking about the /r/atheism type people, the "Facebook Warriors" who feel the need to "educate" everyone with facts/logic whenever someone dare thanks God on their own Facebook page or something equally trivial. And then I think everyone else who is just simply and atheist but doesn't care to really talk to people about it ends up grimmacing everytime people from the militant camp open their mouths.
Interesting parallels here (I think) between atheism and homosexuality, re: "militant".