Well, I've been a writer ever since my pre-school teacher stuck a pencil in my hand and showed me how to write the letter "A." I'm serious. In first grade, my writing journal was full of fictional stories about fictional characters rather than with entries about my birthday party of my favorite food. Before I learned to write, I used to embarrass my mom in the supermarket by constantly blathering to and/or fighting with imaginary people. My mom thought I was crazy. Then, after I learned to write, the imaginary people remained confined to the page and my mom breathed a sigh of relief. "Oh, she's just a writer!" my mom said. Writing has nothing to do with Buffy; it's just a part of me.
For any fandom, I think it is very likely that a high percentage of people who post in forums are writers. Many fans of TV shows (like my husband) are very insecure about their reading and writing abilities. Most of the people that I know in my personal life would not want to write something and then post it on the internet to be criticized by strangers. Those who do post regularly must feel relatively confident in their ability, so it follows that they have some sort of ability. Of those fans having the ability to string together a decent sentence, it makes sense that an unusually high percentage would employ that skill either as a hobby or as a profession. I don't think it means that Buffy attracts writers so much as it means that the opportunity to write essays, arguments, RPG characters, fan fics, etc. about Buffy attracts Buffy fans who like to write.