Just curious. Does anyone know, or is it for the most part random?

I distinctly remember Keith saying something about another account purge this december, but I cannot remember when exactly. Maybe someone can dig up his post? c:
What is an account purge?
Where the staff goes through and deletes frozen and abandoned accounts (accounts that haven't been logged into for 12 months, I believe). That frees up names people have been stalking for usernames and pets. :)