All four were bulls.
The three killed north of West Yellowstone had wandered outside of a buffer zone set aside for bull bison. After Montana officials made several attempts to haze the bulls back into the buffer zone, they were slaughtered.
The fourth killing is more problematic. A bull had wandered into Bar N Ranch grounds and was seen mingling with cattle. That immediately raised a concern, as state officials and stockmen consider the bison to be a threat to cattle as a carrier of brucellosis, a disease that can cause cows to spontaneously abort their fetuses. There has never been a documented case of bison transmitting brucellosis to cattle — though several cases where elk have been fingered as a culprit — but that hasn’t stopped the government from working hard to limit interaction between bison and cattle.
Maybe overly zealous, in this case. When the bull was killed by livestock officials it was over a mile from the herd, according to Bar N Ranch manager Gayle Gavagan, who decried the killing.