You can't see the problem, but you slam on the brakes. That's when you realize that the guy behind was following too close and doesn't have time to stop. You release the brakes, then apply them again, trying to stop before hitting the car in front without getting hit from behind. Fortunately, you had left enough space ahead.
It works! You slow down enough to join everybody else as they crawl past the trouble. After a few car lengths, you see headlights. Headlights? In the fast lane, a car has spun around and is facing into traffic. Fortunately, nobody hit it.
Good choice! As you come over the hill, the cars in the fast lane are all braking, and braking hard. One of them suddenly swerves into your lane, probably because it was the only way to avoid hitting the car in front of him. Fortunately, you had left enough space that he didn't have to hit you.
You continue, starting down the hill. The road is littered with debris from what seems to have been an earlier accident. There are too many cars to avoid the debris, but you are driving slow enough to avoid any damage.
Suddenly your attention is jerked back to your own car. The car ahead is swerving. There are only two lanes and both of them are full. You were already slowing down, but are you about to witness an accident? No. Just the stench of burning rubber and the rush of adrenalin. There was enough space between you and the next car that even when he panics, swerving to a stop, all you have to do is slow, and let him get his car turned back into traffic.
What had happened? In this case, we crawled up the hill to where the police car was parked. Just past the cop car there's a car in the ditch between the two directions. It's upside down! What happened? I don't know, but I wouldn't be surprised if somebody had been too agressive about changing lanes on that washboard surface.