I think the "sudden rise" in Atheism is stemming from the fact that is now acceptable to admit to being one. Back in the days of yore, when you could lose your job, lose your family, be tortured, killed, or excommunicated for saying you didn't believe in God, "out" atheists were pretty slim.
Nowadays society is coming to accept that gee golly - you can be an Athiest - a lot more people are declaring themselves as such. I don't think their numbers have changed at all, but merely their visibility.
![](http://weirdweirdworld.com/images/b3b735627a388bf12a539527da62c26d.png)