Lemmings became the subject of a popular misconception that they commit mass suicide when they migrate.
It's not a mass suicide, but rather the result of their migratory behavior. Their strong biological urges prompt some species of lemmings to migrate in large groups when population density becomes too great. Lemmings can swim and may opt to cross a body of water in search of a new habitat. In such cases, many may drown if the body of water is too wide.