Bison, not buffalo. Edit: note this happened outside of YNP actually.
It’s mating season for the bison. I’m kind of surprised about the grizzly, since bears are most aggressive during the spring. I hope more details come out.
Fun fact, there is a wilderness area on the southern border of Yellowstone NP called the Winegar Hole wilderness. It doesn’t have many trails. Never go there because it is prime bear mating territory!