I worked a season in Yellowstone, surveying wetlands for amphibian larvae. We were in Hayden Valley early in the year, in waders up on a knoll surveying bison-wallows-come-wetlands, and had a group of female bison charge us. I didn't know I could move that fast in waders, running down one knoll, jumping a ditch that seemed too wide to jump, and scrambling up another knoll, only to turn back and see the group milling angrily about the spot we'd just vacated. We later learned they'd been between a coyote, our other coworkers a couple knolls over, and us, and they had calves. They must have decided we looked like the weakest link and decided they wanted some room between the calves and potential predators.And hundreds of bison, they do whatever they please.
No natural predators and up to 2,000, they’re in no hurry ever, or so it seems.
So, they certainly do do what they please, but they sure can hurry when they feel like it too. And I think they take a pre-emptive attack mentality when it comes to perceived threats, which I think they do at least perceive during calving season.