Well, ok, part of the WP issue is that not only is atheism more prevalent on the internet, but atheist culture has generally gotten a lot of strength on WP, in part due to activities that built up anti-theistic argumentation.
Here are the possible explanations I see for the general trend though, as I think one is likely:
1) Contrarianism. Intelligence and divergence from the socially accepted will push one towards more contrarian thoughts. After all, if the current presentation of truth is insufficient, then one will look another direction.
2) The actual superiority of the atheist intellectual position. Note: I am an atheist. I like to think of myself as a relatively researched and smart one. However, I do think it is a fact that atheism is a better intellectual position. I recognize you don't want too much debating on the matter, so I won't elaborate too much.
3) Cultural identification. People with AS do not identify with the mainstream due to their differences, while theism tends to. This leads them to form an identity in opposition to that mainstream. People with AS tend to identify a lot with science, making them have something in common with the atheists. Smart people are in general more likely to be atheists, with some academic disciplines dominated by atheism. Also, it is possible people with AS have a stronger connection to the internet, which tends to have a strong atheistic tendency in it.
4) AS producing rationalist/reductionist proclivities that tend to be disfavorable to religious claims and their justification.
5) Intelligence producing greater variance in beliefs, thus producing more atheists as a percentage of the population, as intelligent people are unlikely/unwilling to accept things as granted.
Now, I am going to say that I favor 2 and 3 more heavily, and even recognize the two as somewhat feeding into each other.