I saw a video explainer. Apparently the truck honked a bunch of times and that upset the bison.

I'm no bison expert but it looks like that was not enough, at all.

My ex used to work in Yellowstone, and they closed her entire building because a bison was taking a nap across the street. The management was worried that customers would get gored in the parking lot, so they shut down the entire lot and building until the bison was gone.

There was another instance where a calf (they usually called them red dogs, because they tend to look like dogs from a distance) was sleeping on the road between her job and the employees’ living quarters. They cancelled the shift change (and everyone currently working had to pull a double shift) because it was too dangerous for the staff to swap out while the mother was likely nearby.

All of this is to say that 100 feet wasn’t enough. The recommended distance is more like 200-300 feet, minimum. Bison are extremely territorial, especially during mating season.