Don’t try to apply logic and sense to the herd, for their motivations are so simple and feeble minded an intelligent creature can oversubscribe meaning to their actions.

They move to the right because the ass they follow moved right. The move to the left because the ass they follow moved left. They jump from the cliff because the ass they follow has jumped.

Their only motivation is fear of the shepherd dog nipping at their heels. If there is no shepherd dog they simply follow their stomachs through the grass.