I think it's a couple of things. In order of magnitude:
1. THR has now been in existence long enough that it generates a significant number of Google hits when people perform a number of search terms.
2. Oleg's photo's have gotten wide play. His intended effect to convey RKBA clearly, concisely, and very "in your face" stirs up people. Those photos have also gotten spread about and gotten lots of play in many non-gun forums. That leads people back to THR.
3. THR people and other gun-boards have gone "trolling" at places like Democratic Underground, the more successful ones posing, or legitamately, as "pro-gun liberals", and if they hot-link right into the DU thread du-jour, the web-admins at those sites can see this, and the places these reverse-trolls come from is known. (Same for when an RKBA poll get's "freeped" too&)
Some of the people are legitimate pro-gun liberals.
Some of them are high-school and college kids stumbling upon THR as they perform research on position papers for or against gun control.
Some of them are just trolls.
I do agree with those conservative and libertarian posters who think that the self-deterministic nature of RKBA, and all it implies is somewhat inconsistent with collectivist/leftist thought, but I'm not going to be rude about it. And I definitely agree that everyone being right of center on a board is boring. As long as it's not disruptive, or devolves into meaningless flame-fests having a breadth of political opinion is interesting.
I also think THR's strict moderation, lack of any advertising etc. does make the site a bit more civil in tone, and there are a rare few anti-gun people who really want to dialouge and feel the most welcome at places like THR, where other more "rambunctious" boards probably make them feel like a feminist walking into a strip-club...