Found this on SynthGear:

As each metronome comes to a stop & reverses direction at the top of its swing, it transfers energy to the platform. This energy transfers through the platform to each of the other metronomes.

The energy transferred to the platform from each metronome starts out of sync, but the waveforms of the energy interfere with each other, and at the location of each metronome you end up with negative or positive waveforms which in turn have an effect on the interference pattern. This keeps going until there is a balance – this balance is only achieved when all of the metronomes are in sync. The interference pattern is now a standing wave, which keeps the metronomes locked in phase.

When the whole contraption is put on the table, the table absorbs all the energy, reducing the interference patterns. You can see this phenomenon in many different types of oscillating systems, which can cause some major problems, especially with traffic on bridges and wind around buildings. You could also do the same thing with ticking watches, or even watches, which will eventually tick in time if places on a thin sheet of cardboard help up something like the cans in this video.

Of course if you’re a synth fan, you know that waves like these have harmonics, and the synchronization will also work with multiples of the fundamental frequency. You could presumably set some metronomes to half-speed to double-speed, and they would all tick in time, with different rhythms.