Per the "internet", the buffalo are running away from the Yellowstone Caldera. I have yet to see any corrobative statements. They might be feeling frisky and playing also.
http://www.youtube.com/watch?v=RiexHmbJlp4That is unusual considering that the buffalo aren't concerned at all about the vehicle taking the video, whilst they are running away from the volcanic vent/gassing event on the ridge they have come down from.