Auto ban usually occurs (someone correct me if I'm mistaken) when a user that is banned attempts to log into one of the other UID's via the same I.P. that there previous alt had.  It's a great way to catch out previously banned users who may never have been caught out any-other way.  (it's been known to catch some big fish that way    ) so it's important to notify the mods of ban evasion in case they are able to mask their I.P.'s.