Yellowstone may blow. And, I've heard the hypothesis that it, and the faults in CA/along the west coast, are interdependent. But...
Bison run. They just run. Pretty frequently, for long distances, and for long periods of time.
Or, it could have been wolves, some tree hugger, a dog, a coyote, a sage grouse that got flushed offering a good excuse to run. They've been running since they had legs.