Not sure how it is any more strange to see someone actively speaking that religion is is false, when people all over the world actively speak that particular religions are truth (the only truth in many cases).
As for the motivation it obviously depends on the person. But for the most part, I think many of "these people" just think religion is transparently bogus and say what is on their mind.
A lot of people on this board speak out against the "PC" culture. Particularly with race...there is the idea that whether or not you choose to see it there IS oppression and bigotry everywhere and as a result we all need to be super sensitive to racial issues, b/c racial equality and the struggle to finally get to some mythical promise land where we achieve it is basically sacred. So you dare not say something that may offend someone. A lot of people believe that there is a big problem currently with race. A lot of people don't see that at all. MOst people just ignore or don't speak on it. Some do. Blah Blah.
When it comes to religion these discussions often can turn into "I have faith but respect those who don't" and "I don't have faith but respect those who do." A giant circle jerk...b/c it's just fluff and leaves out the fact that a large portion of people (if speaking their mind) will say "it's pretty bogus and frankly I don't personally have 'respect' for people's belief in ancient stories about things that I absolute think are false."
I'm not, and nobody I know is, saying that anyone should abolish religion. But I certainly don't have to go around just acting like it is somehow entitled to my personal respect. I respect people, and I respect people's rights, but I don't see how it's somehow taboo to admit that if I'm honest I truly don't have any respect of belief in religion. It isn't nasty, it isn't targeted, and honestly it doesn't really affect my opinion of people since so many people are (or claim to religious). But religion itself, it isn't sacred to me. It's silly. I don't "respect" the actual belief of this stuff. I don't feel the need to walk on egg shells. And it is that simple.
So the answer to the motivation question is simple...it is how I feel.
PS: That is almost unreadable but I can't be bothered to edit it. You catch my meaning, I'm sure.