It's no mystery, r/jailbait is the 7th result for "jailbait" on Google. The people who go to r/jailbait from a Google search are just hitting the first link that looks like it might be what they want.

But from this viewpoint it makes sense that most people with the alexa toolbar installed go to reddit for jailbait...

Does that mean webmasters over-represent the people on reddit who are interested in underage porn (okay, borderline)?