I think part of the answer is transient bottlenecks and a backward propagation of the traffic waves they create. They’re caused when a large number of cars merge onto the freeway at an onramp, slowing down the right two lanes; when some jerk cuts someone off, causing a momentary slowdown; when there’s something interesting on the side of the road that people decelerate slightly to look at. Of course, actual bottlenecks obviously cause a backup as well.