This is because Twitter has announced that they will begin cleaning up and purging inactive accounts. According to Twitter’s definition, an inactive account is one that has not been logged into in the past 6 months, so if you haven’t and still want to keep your account and username active, you should do so by the 11th of December.
Failure to do so will result in your account being deleted and your username being put back into the pool of available usernames to choose from. According to Twitter, “We encourage people to actively log in and use Twitter when they register an account. To keep your account active, be sure to log in and Tweet at least every 6 months. Accounts may be permanently removed due to prolonged inactivity.”
The upside to this purge is that if you’ve been meaning to change your username, there is a slight chance that following this purge, you might be able to finally get the username of your dreams.