Why You Know This "Fact":
Calling people a bunch of lemming is like calling them "sheeple." The small, furry lemmings are known for being mindless followers who will even commit mass suicide by jumping off cliffs. They've shown up in cartoons, commercials and video games doing exactly that.
Too bad none of it is true.
This rumor was probably started in the early 19th Century. Scientists would see sudden bursts of the lemming population that would just as suddenly shrink, and they couldn't figure out why. In 1908 a man named Arthur Mee decided that "mass suicide" seemed as good an answer as any and published it in his The Children's Encyclopedia.
Arthur Mee refused to shelter children from the harsh realities of the shit he totally made up.
Mee concludes that the self-inflicted almost total annihilation of lemmings is "sad and terrible, but if the dismal exodus did not occur lemmings would long ago have eaten Europe bare." This means that Mee believes that lemmings kill themselves out of an instinctual drive to preserve Europe at the expense of killing themselves, and everyone reading along at home said, "Yeah, that sounds right."