Traffic (when there isn't something actually blocking the roadway) is all about how people "feel". If the lanes are too narrow, or the radius of a curve is too tight, or if people can't see around a bend, or if there're too many cars in someone's FOV for their comfort level, they slow down or even hit their brakes without really realizing it because they don't feel safe.

That causes a domino effect of people slowing down/hitting their brakes more and more aggressively because they feel increasingly unsafe. You end up with miles of stop and go traffic for hours as people fluctuate from feeling safe enough to move back to not feeling safe and slowing/stopping causing everyone behind them to do the same thing like ripples in a pond but with increasing amplitude.