I'm thinking the bison sensed the ongoing natural disaster that is the modern world and instinctively moved in a direction away from it, which was deeper into the park.

The second video is less visually delightful than the first but the guy said there are many small earthquakes every year in the park, so I'm guessing tremors wouldn't startle them overmuch.

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1986 300SDL, 362K
1984 300D, 138K