On a macro level, many animal species act like the made-up lemmings. Most of the time, predators and food supply keep the critters in check:
- With lots of food and few predators, the animal reproduces quickly.
- As the animal multiplies, they begin consuming the food more rapidly, and predators begin to increase
- Eventually, there are more critters than food, and more predators than critters; the critters starve and/or are eaten.
- With fewer critters, the predators die off and the food regrows, and the cycle begins again.
But what if there are no predators? Islands often have very small, specialized ecosystems; it's not difficult to imagine an island populated by nothing more vicious than a blue-coated critter with green hair and a long nose. In that case, the only limiting factor is food. But what if the food remains plentiful until a certain point, then almost overnight is gone? Rather than a slow, constant, more/less cycle, the food is almost a binary switch - food today, none tomorrow. This would have a huge impact on the survival of the critters. As their population grows, they come closer and closer to destroying their food source, wiping out the entire population. If their population grows too much, they may wipe out the species entirely.
So, nature installs a switch. The critters are natural introverts; the more time they spend near other similar creatures, the more stress-induced chemicals build up in their brains. As their population booms, more and more critters accumulate chemicals. At first, the chemicals have little effect, but eventually they cause the critters to essentially go insane, their desire to be away from other critters overriding their self-preservation. The critters will begin chasing other creatures in an attempt to drive them away; a stampede of critters begins, sweeping up a large amount of the population, and terminating in the ocean. The more stress chemical acquired, the stronger the desire to chase, and those initiating the chase are most likely to survive.
With a significant portion of the population gone, the rest calm down, and the cycle begins again:
- Critters reproduce
- Critters overpopulate and begin generating stress chemicals
- Super-stressed critters chase the rest into the sea
- Surviving critters begin repopulating
I realize this answer doesn't fit the criteria of followers and leaders, but it is how I imagine "suicide lemmings" would act.