Why did the Bison cross the road???

All traffic stopped as an entire herd of bison crossed from one side of the road to the other. Wildlife has the right away and traffic can get quite backed up.  Some cars would go when there a few seconds break before another bison crossed.  Baby bison would be sandwiched in between larg adults when crossing. 

We were on the motorcycle when we stopped and I got off to take some pics. from across the road once they were all on the other side.  However, one big bison (in the foreground) made snorting growling sounds as it started heading back across the road.  I hid behind someone's car who was parked and then ran to the bike, got on and we moved on.  There were bison all along the roadside and at one point, we were so close to one grazing along the roadside that when it heard the bike, it's eye rolled toward me as he was still grazing and he snort growled. I told John to hurry up and move it, he was very close.  Luckily, he was more interested in eating grass.