If they were walking away, Im guessing she turned to meet the bison head on?
Ive recently been 40 feet from one, but we were on a bus, he stopped because he wanted the road to himself for a few minutes..
otoh. I wouldnt get within 500 yards of bison unless there was a barrier between us strong enough to stop a F-350 quadcab dually doing about 35 mph.
Anything getting too close this animal's ‘’space’’, say fifty yards or under and the animal will respond accordingly. Best to stay much further back.
In the video the animal first reacts by charging at what it perceived as the threat, the humans. And then it veers off to it's left in an effort to get away from them.