https://daily.jstor.org/the-science-of-traffic/

Often the cause of a jam is evident, as in the case of a crash or construction. But the biggest threat to traffic flow is actually a slow vehicle. Drivers get caught in what’s called a slow-moving bottleneck, caused by a vehicle moving slower than the flow of traffic in one lane of a multi-lane road. Even though traffic never stops, a traffic jam forms as if an actual blockage had occurred.

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.