White-bearded wildebeest decided en masse to make a u-turn while migrating to the Serengeti. It was some sudden bursts of rain in the Mara main that had made them decide to cross back over the Mara river in an easterly direction from the Mara Triangle.
These large herds of migrating wildebeest have a chaotic herd structure. In cases where herds do not have an obvious leader, I have actually seen calves leading the adults into crossings of the river. Such leaderless herds often choose an impossible crossing point.
The only way they could possibly succeed in scaling the high exit bank of the river would be to climb on top of each other. The result is that wave upon wave of instinct-driven wildebeest would just keep coming, maiming and killing each other in a chaotic frenzy .
Five wildebeest succeeded in crossing at this point while approximately one hundred were killed or maimed by other members of the herd and about five or so were taken by crocodiles. On the other hand Burchell’s Zebra, with a far more intelligent stallion as their leader, did not suffer a single casualty!
The stallion just abandoned the crossing, thereby avoiding a lethal stampede.