Lemming

They're the cute little penguin-like bird-like things that waddle en masse over cliffs together in this Jim Jonesian mass suicide every so often. One starts the process after being dumped by his girlfriend, fired from his job, and losing his car keys just one too many times. The others all follow. It happens in nature and of course, Wall Street adopts the aphorism. On the Street, it applies to investors who note a given stock as suddenly being bought up--the price spikes. Others have FOMO and buy in. And more buy and more buy and more buy, none of the new buyers knowing (or often even caring) why it's going up. They just care that it's going up up up. And they keep owning and/or buying until they stop. And then it goes down down down and investors who bought the momentum stock are now dumping it over a cliff as they follow each other like lemmings, mindlessly committing portfolio management suicide by basically just doing what everyone else does.