[steamer25]

It is interesting but not inexplicable. For instance take a scenario where one driver sloooowly passes another on a two lane road. This effectively clotheslines the road and causes anyone who would drive faster yet to pile up behind the two cars--maybe even apply their brakes.

Again, consider a sudden lane change that cuts off someone who's already in the lane being entered.

Yet again, consider that most on-ramps are essentially a reduction of available lanes.

Any time one car slows down for any reason, inertia kicks in--they do not immediately return to speed.

In this case, small variations in speed mean that some drivers eventually catch up to others. Then they slow down a bit which reduces the space behind them. The problem is that the road is nearing peak capacity and has little tolerance for deviations.