I’m with @Seek that they are a reaction to the Jesus/life/religion in your face billboards. On one hand I’d like atheists to be able to say “we” don’t do that, but at the same time I see how the only way to show the theists that putting up billboards is annoying, and so in your face to people who don’t think the same way or believe the same things. Although, in the end I don’t think it shows those theists much of anything. They don’t get introspective about it, they just feel they need to fight more. Be louder. Plus, they can tell the flock the country really is trying to take away their religious freedoms. It feeds into their propaganda machine.
Mind you, I don’t think most religious people are trying to put up billboards and frighten their followers, but enough are that it’s a thing.
I think the intent of religious billboards is to convert people and get the people putting the billboard messages up into heaven. I don’t think the intent of the atheist billboards are to convert people, but maybe I’m wrong?