Bison celebratory running

I call it "celebratory running" because the bison appear to be running out of sheer joy rather than fear of something.  I don't know if this phenomena is seen in other bison herds but my herd appears to do it every month or so.  The above video captures a brief moment of the beginnings of one of these runs.  It's unfortunately the only video I've been able to record of the event. 

The process starts when one or two bison begin energetically chasing each other in close proximity to the other bison.  The other bison will watch with interest and several more will then join in the chase as it weaves through and circles around the herd.  More and more animals join until eventually the entire herd is a swirling energetic crowd of bison moving across the pasture at a fast pace.

To many people celebratory running can look a lot like other types of running, but there are differences.  Bison in a herd running out of fear, for instance, will focus more on things outside of the herd and less on each other.  The animals stay in a tight group and tend to act in unison.  With celebratory running the bison are focused mostly on playful interactions with each other.  The animals are grouped more loosely with each animal doing it's own thing.  Some bison may make mock charges at each other, while others will sprint, stot, or buck. It's a high energy event and always fun to watch.