CGP grey did a video on it: https://www.youtube.com/watch?v=iHzzSao6ypE
Basically one car breaking too much will make the following brake even more and so on until one stops and there's a jam. There is no clear reason like a road blockage or an accident, just compounding slow down.
The irony is that nowadays, something that is universally considered non-human is able to do these things, arguably better than the average human.