Earlier quoted context omitted.
Did you not hear the warning tones trigger before the accident occurred?
Isn't that because the Tesla can see the brake light on the car two cars ahead? As soon as the red car swerves slightly to the right, and the brake light becomes visible, the Tesla brakes. Moreover, the speed of the cars before it are suddenly slowly down.
Not the brake light: the speed differential between that car, the car one car ahead, and the car with the radar.