Something that might interest you. The Millenium footbridge in London had to be closed, tweaked and tuned after engineering design issues were discovered. A subtle interplay and feedback loop between people walking in lockstep and the bridge's resonant frequency caused eventual high amplitude oscillations and eventually sickness in humans and stress in the bridge.
Fascinating stuff!
It is a stronger effect if the people know each other and are having a conversation, but it may well be that if most people have their noses buried in a phone it happens as well - maybe audio plays a part and subconsciously hearing footsteps they sync up their footsteps.