At least in the UK, it would very much be the fault of the driver in front, especially if there was that much CCTV footage available (there won't be, which is why dashcams are important - the US's obsession with blanket CCTV coverage scores a point here).

The second car had left a more than adequate stopping distance. The Tesla changed lanes close in front of it and then immediately braked as hard as possible, deliberately. The driver of the Tesla should lose their driving licence.

The drivers following the second car weren't leaving enough distance or paying enough attention.