There is no evidence that lemmings would deliberately jump off cliffs to their deaths. This so-called "unreasonable" and "compulsive" behavior was all made up by an award-winning 1958 Walt Disney television documentary "White Wilderness."
"White Wilderness" was filmed in Alberta, Canada. The filmmakers, led by principal photographer J.R. Simon, staged the supposed mass lemming suicide with the use of carefully planned angles and tight film editing techniques. In the film, the lemmings had to be imported from other places, since Alberta is not their natural habitat. The staged mass lemming suicide only used a few dozen lemmings, but the film edits were done to make it appear that there were more lemmings that threw themselves off a cliff.