Not surprisingly, slowdowns are the leading cause of traffic.
A car behind the slow vehicle has to brake and slow down, forcing the car behind to brake, and so forth. The slowdown spreads upstream of the slow vehicle in a wave, called a shockwave. Now, the car behind the slow vehicle will try to get around it. In tight traffic, a desperate car trying to pass the slow vehicle will inevitably pass close in front of a car in an adjacent lane. That forces the driver in that lane to brake, propagating a shockwave in that lane as well. These slowdowns will propagate sideways through other lanes as cars try to evade the slowdown and then backwards as new slowdowns are created.
Food for thought for our stop-and-go traffic caused by our public transport as well as the much vaunted 30kph speed limit of Davao City.