Robert Gonzalez at io9 explains: "If you place 32 metronomes on a static object and set them rocking out of phase with one another, they will remain that way indefinitely. Place them on a moveable surface, however, and something very interesting (and very mesmerizing) happens.
The metronomes in this video fall into the latter camp. Energy from the motion of one ticking metronome can affect the motion of every metronome around it, while the motion of every other metronome affects the motion of our original metronome right back."
In the comments for the metronome video, someone mentioned being reminded of the way people walked on the Millennium Bridge: