There is a long story about how this happens.
One morning I am passing Hayden Valley at about 11:00am, I saw crowd gathering and lots of cars parking along the roadside. It's a grizzly mother with three cubs trying to cross the river and get a carcass. That is an elk carcass killed by three wolves last night. The grizzly family is too far and beyond my reach. So I didn't stay long there and head for Yellowstone Lake for a cruise. Unfortunately, the cruise is full and so I return. It's 4:00pm this time and the same crowd is still there in Hayden Valley. This time the grizzly family occupied the carcass. A coyote is coming here but can do nothing. I decided to come back again and check at around 7:30pm. Well, this is a good decision. I saw a wolf by the river watching the grizzly family enjoying the carcass. Then two coyotes are coming and howling. It's loud and clear. One of this wolf's leg is injured when they are killing the elk. So he sit by the riverside long time. Maybe he is thinking how to get a share of the carcass from the grizzly family. These two coyotes seem not able to cross the river (Alum Creek) and so they go around it. I don't know why they want to cross the river though. Suddenly, this wolf is crossing the river. But he has no courage to get closer to the grizzly family as the grizzly mother is watching him all the time. So he went on to the same side as the tourists stand. After some time the two coyotes are successfully crossing the river from somewhere and meet this wolf right in front of everyone. They seems trying to chase out this wolf so that they may have a better chance for that carcass. The funny thing is, they succeed and the injured wolf ran away in front of the crowd. In fact, the passage is left for the grizzly family in case as rangers asked. It's used by this wolf in the end.
No one knows what they are thinking. The ranger said he has never seen this in Yellowstone, where he has spent over 10 years.