Most accel/stall occurr or are demo in horizontal flight as in a dearture stall/spin. They are very infrequently seen by ground observers (or pilots for that matter) in a nearly vertical nose down position. What appears as a spiral in the video is most likely a spin but the entry, because it was nearly vertical, makes it appear different. The explanation of flap/radio was very vague....what is the specific evidence? Again, no way to know for sure, but what is the probability of some kind of interference issue at that most likely point you would expect a stall/spin to happen in this circumstance. PROBABLE CAUSE.