Bridges wobble because walkers sync their steps

The London Millennium Bridge had to close two days after opening in 2000 because pedestrian footsteps set it swaying. Engineers discovered that people instinctively synchronise their gait with a wobbling bridge, amplifying the oscillation in a dangerous feedback loop. Two years of modifications — adding 91 fluid-filled dampers — were required before it could reopen.

Source: Arup: London Millennium Bridge Pedestrian-Induced Vibration Study (2001)