Anonymous userbase + higher visibility + other, smaller but otherwise equivalent boards. Same thing that happens to most anonymous boards, really.
This is a dangerous combination, because the people who have been around for the longest don't have any post count / reputation / etc. that would present some feeling of obligation to uphold, so the bar to leaving one place and beginning to post at another is much lower. No announcements, no decisions required, no new account to set up, no profile links or e-mail addresses to change.
New people discover a board and import their own brand of humor unfamiliar to the early adopters and long-time regulars, and possibly also bringing to the board their own personal in-jokes, as occasionally one person will find a new site and share it with their friends. Those who used to be most familiar with the site will find themselves alienated by these new users. Plus, the newcomers have an advantage over the existing members here because they have the ability to read through old threads, gaining a shallow familiarity with the site's culture and terminology.
This is a constant process, and it will inevitably happen on any small to medium-scale forum on the internet, but it's most prevalent and noticeable on anonymous boards focusing on original content, because those are the ones where the users tend to adopt part of the board's culture as their own.
It's been suggested that the internet operates on a much different scale of time than the physical world, thanks to the instantaneity and accessibility of broadcast communication. I believe this userbase shifting tendency takes place offline as well, with cities analogous to web forums. (Indeed, considering the original meaning for the word "forum" being what we might regard nowadays as a mall or plaza, serving as a meeting point and cultural center, it's a fitting word for the centerpoint of internet culture.) As goes a typical ice-breaker conversation question of what one might expect of a notable historical figure were he introduced to modern times, I expect that he would feel every bit as alienated as any early-generation DQN poster would feel when looking at how the board has evolved over the years it's been in place.