There is something about being human that people walking in a group tend to end up walking in lockstep. Not a military marching level of synchronization, but far more regular than random. I first noticed myself doing it as a child and since then have become very aware of it happening around me.
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.
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!
Next thing I noticed, which kind of freaked me out, was on the walk over bridge that connects Waterloo with Waterloo East. You enter with the herd, and its kind of pure random noise. What is interesting is that by the time you all get to the end of the walk way, more often than not, every one would be walking in pure synchronisation, like a marching army. They became one. Almost like Borg.
Noticed something new recently. So many people now have their noses buried in smart phones or tablets, fingering away. Even less chance of breaking the barrier. They will never see you smile...