metronomes (or "pendula") when on table, oscillate with random phases, since that is how they started and they are "uncoupled" (no energy/information flows from one to other so they do not "know" each other.) When they are all together on the cans, notice that the cans themselves oscillate little, providing coupling/information crossover. which forces "synchronization" in periodic systems (discovered by Huygens in 17th century).
a useful book: "Synchronization: A Universal Concept in Nonlinear Sciences " by Arkady Pikovsky, Michael Rosenblum and Jurgen Kurths.
Synchronization is a common phenomenon in physical and biological systems. We examine the synchronization of two (and more) metronomes placed on a freely moving base. The small motion of the base couples the pendulums causing synchronization. The synchronization is generally in-phase, with antiphase synchronization occurring only under special conditions. The metronome system provides a mechanical realization of the popular Kuramoto model for synchronization of biological oscillators, and is excellent for classroom demonstrations and an undergraduate physics lab. ©2002 American Association of Physics Teachers.