Final answer:
A bison may charge across the plain in response to a perceived threat or to establish dominance within the herd. Seeking shelter or searching for food usually involve less aggressive movements.
Step-by-step explanation:
When a bison charges across the plains, the behavior can be attributed to various reasons. They could be responding to a perceived threat, which is a common reason for many animals, including bison, to charge. Charging could serve as a way to deter predators and protect themselves or their offspring. It might also be related to establishing dominance within the herd, as animals will sometimes assert their strength to maintain or change the social hierarchy. While searching for food sources or seeking shelter from predators could also cause a bison to move, it's less likely that these activities would be done in the form of a charge. Instead, they would be more methodical movements. Without additional context on the situation, it's challenging to pinpoint a single primary reason; however, responding to a threat or demonstrating dominance are more likely associated with the act of charging.