You see, what happens is that the person behind the first rubber-necker breaks slightly harder than they need to to avoid hitting them, and then the next one breaks slightly harder again and so on…

This has a name in traffic management circles, which I've completely forgotten as I'm not doing political work in that area any more.

One of the Police Stop videos actually has a very good example on it, filmed mostly from helicopter.

Oh, I remembered - it's called a compression wave effect or something like that. Makes sense.