The Running of the Bulls
It is not Pamplona, Spain but the bulls (and cows and calves) do run in Yellowstone. On a number of occasions we have seen large numbers of Bison run and, no, there is nothing chasing them. In this video, a large herd of maybe 500-700 Bison run from a high meadow down and through the Lamar Valley after crossing the Lamar River whose swift current can carry a little calf away.
The last time we witnessed this event, the Bison were coming over a hill along side a little lake, then up another hill and down the other side. They could not see where the other Bison before them were going and yet seven separate groups, with a couple minutes pause between each, passed us. It was very much like they all knew where they were supposed to go.