It happened at Otter Creek in Yellowstone National Park in October. The female bison (known as a cow) wasn’t feeling very well. You could tell that just by watching her. She was lethargic, and never moved very far away from a small meadow she had chosen. Had she felt better, I would have expected her to join the bison herd less than a mile away.
It was also pretty clear that the wolf knew she was vulnerable.