Those probably aren't as genetically pure as the Yellowstone area's bison. Bison were bred with domestic cattle, and a lot of the wild population have cattle genes that may make them more docile. There were also escaped cattle that may have mated with bison.
Nearly all ranched bison have domestic cattle ancestry.
Bison are rather unpredictable though. I don't know about two speeds. I've waited as a line of bison were crossing the road slowly. I've never seen a stampede (I suppose Dances With Wolves was pretty accurate). I have driven by and saw bison gently galloping. Here's one running along a road in Canada. I don't think the driver is doing anything to provoke it, like stop in front of it.