The "Tag Fairy" is a person who is a member of the disboards who can change your avatar to read a saying that you personally have posted sometime on some thread.
Not sure if anyone know who it really is, but I like the mystery myself.
Anyway, if you see sayings under the DisName that are in color, the Tag Fairy did it (or Genie during the games). You yourself can only add in the black colored ones.
Not sure what prompts the tag fairy to leave a tag. It doesn't seem to have anything to do with time here or postings you have. Just when you say something really clever or stupid, TAG, you get one.