Where liability falls when an external party startles a creature after the restraint is removed but before it departs?

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If a person opens a cage and unties a horse, and both remain standing, but a third person then startles them causing them to leave, the liability rests upon the one who startled them. This is because the startling action is the most specific cause and thus holds the responsibility, analogous to the one who pushes versus the one who digs.