The problem is in this case was that they were too low to be able to recover no matter how good the pilot's spin recovery training was.
They were too low and too slow, which goes back to poor piloting training and/or lack of experience.
I watched the video quite a few time. At the 8 second point in the video he applies full right aileron coupled with what looks like full right rudder with what appears full up elevator, then the pilot unloads the aircraft at the 9 second point, still with full right rudder and aileron, then at the 10 second point, the pilot again applies full up elevator resulting in the aircraft going into an unrecoverable spin because of the low altitude.
From what saw on the video, it was a pilot induced spin. Now If it was accidental or not I don’t know. However, if the pilot had proper training, knew how to induce a spin and then recover, it would more of a case of suicide rather than an accident.
So in my opinion, the probable cause was an accidental spin due to poor pilot training and lack of experience. But before I would positively state that, I would need to know the pilot’s experience, total time in type, total time and training records.
A damn shame no matter how one looks at it.