Could be several reasons. For one, outright hardcore pornography is still illegal in South Korea. The site may have caught the attention of the authorities and they nuked everything preemptively.
They also didn't like foreigners sharing information from the site, like direct links or the passwords for the downloads/archives because it would draw more attention to the site. They wanted to keep it low-key. Since more and more people have been discovering the site and sharing stuff from there, it may have pissed them off enough to nuke everything.
I'm just speculating here, though.