In order to guard against imposters, your nick cannot contain the name of any member of network staff. In most cases, this rule will not cause any problems, but there are some tricky cases. For instance, because a member of network staff is named Atri, you are not able to register nicknames like Patrick, Patricia, or Atrium.
In the case of your nick, the word Trevor is the problem. See
http://www.geekshed.net/2012/03/choosing-a-nickname/