Give it about two minutes.
A Youtube comment explains the phenomenon thus:
Each metronome is contributing horizontal force each time it swings, from the momentum of its weight. If two metronomes move opposite directions they will cancel each-other’s force. But if they are not perfectly synchronized, their respective directional force will be translated to the board they are standing on. If several are moving CLOSE to the same direction at the same time, they will influence the board’s motion more. Soon they are influencing other metronomes to move in the same direction.