This is the greatest mystery of the DIS!! I'm guessing it has nothing to do with the number of posts.. because I've seen DISers with thousands of posts and no tags... then I've seen DISers with a few hundred along with a colorful tag!
MY thoughts on how to get a tag:
Hang out in the right forums.
Say something witty, funny, or quote-worthy!
If you figure the secret recipe out, let me know!
Good luck!