You got me there. I know you are so glad
But I do believe the "myth" has some basis because, apparently, every four years or so, the population "booms," forcing them to seek territory beyond their usual habitat. In mountainous areas, there will, inevitably, be a cliff which the lemmings can't avoid. Though it's not "purposeful" suicide, there are some who will fall off the cliff and die, owing to sheer numbers.
This happens to wilderbeests, too, in their yearly migration. Some cannot ford the river which is "in their way." As a result, they are eaten by crocodiles.
You're right, some of the little critters get shoved off cliffs and ledges as they try to disperse into new habitat - so I guess it's more murder than suicide. Lemmicide?