Or because they've been driven from the rear and prompted by by a lead decoy to run toward a natural Ha-Ha, a drop hidden by the curve of the land.

https://en.wikipedia.org/wiki/Buffalo_jump

https://en.wikipedia.org/wiki/Ha-ha

There are second hand accounts from indigenous people, excavated bone sites with tools and spear tips, but reportedly no first hand recorded observations from Europeans of buffalo running off cliffs.