Elk bites can be pretty serious.
A moose once bit my sister.
They can. Deer too. A few years ago my dogs broke a fawn walking down a grassy hill. The dogs, trained not to chase animals, stood motionless. The fawn ran straight at me, don’t think he actually saw me, it was just the trail, and I barely got out of his way. Wouldn’t have wanted to have been hit. And mom was sounding off in the woods about 50 yards away. This sounds like an unfortunate accidental incident, but I’m amazed more tourists aren’t injured at Yellowstone. I guess the animals get accustomed to the annoying two legged things.