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There's a pretty good description at http://salt.uaa.alaska.edu/dept/metro.html . It happens because the wooden base is placed on two rollers (pop cans?), which couples the independent systems (metronomes). When a pendulum swings to one side (e.g. left), momentum pushes the base in the opposite direction (right), which then pushes all the other pendulums back in the original direction (left). This action tends to be self-reinforcing, because greater synchronization between pendulums leads to even stronger synchronizing force.

The effect is present even with a fairly rigid base (see http://en.wikipedia.org/wiki/Odd_sympathy), but putting it on rollers makes it more dramatic.
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