Bison don't see very well past forty or fifty yards so anything that gets too close is perceived as a threat and they respond aggressively. That's how they've survived for thousands of years. That was one bison and one human. Imagine a herd of them.
“Imagine a herd of them.”
Especially when indians got close enough to spook them off a cliff, instead of using a bow & arrows to bring some down, for winter meat and hides.