But that's not what happened at all.
Instead, it started with what's called a "bear jam," the park's term for when visitors stop their cars to gawk at a bear—in this case, a family of bears grazing on a hillside near a bridge.
Suddenly the bears, including a mom and her yearling cubs, took a wrong turn and ended up on the bridge along with the curious human onlookers. Their instinct was to get away from the people as quickly as possible, but the people got frightened and blocked her, says Kerry Gunther, head of bear management at Yellowstone.
"It's obvious she gets a little nervous as she's trying to get across the bridge," says Gunther, who was not present but watched the video and spoke to a park colleague who observed the incident.
"The bear was not after people—it could have easily caught anyone it wanted," he says, adding that the video shows the bears trying to get around the people, but being thwarted because the humans kept moving.
"Pretty much all the events [in the video] were influenced by human behavior."
Gunther offered some tips on bear behavior and human etiquette around bears in Yellowstone.
Is this sort of incident unusual?
Yellowstone gets millions of visitors from all over the world, Gunther says, and most know nothing about bear behavior. In this case, "People got out of their cars and then panicked, rather than keeping a clear head," he says. "You should never run from a bear." Running can trigger the bear to chase when it might not have otherwise.
What should people have done differently?
People should stay at least 300 feet from bears (100 yards), according to Yellowstone regulations. Gunther recommends that visitors watch bears from their cars, so they can just drive away if a bear approaches.
As for the people in the video, ideally it would have been better for them to stop and let the bear pass instead of running alongside it, he says. "All the people could have gone to one side of bridge and allowed the bears to have an escape route."
What's going on with bears at this time of year?
At this time of year, bears have recently emerged from their dens and are starting to feed on spring vegetation, he says. Mothers are still with their cubs, but it's around this time that the yearlings strike out on their own.
Gunther adds that black bears in general aren't very aggressive, and in general, black bear attacks are really rare.
Typically, bear attacks occur later in summer, when bears are trying to put on weight for hibernation. Because they're so focused on feeding, they may be less wary of humans, and thus more likely to have close encounters with them. (See “Maulings by Bears: What’s Behind the Recent Attacks?“)
What should people learn from this experience?
Visitors to Yellowstone should enjoy and watch wildlife, but in a safe way, he says. For one, he suggests visiting the park's website, which has information on how to act around bears.
"We have to figure out more effective ways to get the message out about bear safety and bear-viewing etiquette so when people get here, they know how to behave around wildlife."
Did this harm the bears in any way?
The incident didn't have any lasting effects on the bears themselves, he added. "It was obviously stressful at the time, but in a place where we pump over 3.5 million people through each year, bears encounter a lot of people," he says.
"They have to habituate to the presence of people as a way of surviving in a human-dominated landscape."