This is the process of being thrown off a map. If a server unexpectedly exits, all current players and observers will be kicked. Administrators can kick individual players as they see fit. Behavior liable to get a player kicked includes: cheating, deliberate teamkilling, swearing or abusing players in messages.
Administrators kick players with /kick:
/kick <playername> <reason>
The reason is shown to the kicked player, so keep it polite.