"Lemming" is a term for a MUSH or MMORPG player (or group of players) that rushes to participate in a scene, usually because they expect to find personal gain but sometimes simply because they're bored. The term is derived from the rodents of the same name who are widely believed to follow each other anywhere, including to their deaths.
The lemming phenomenon can be mitigated by implementing a good screening process (such as application) to improve the quality of players on the game. It will never fully disappear, though, as game plots where characters search for artifacts to gain power will always be popular.