Possibly a “petit mal” seizure. I’ve had two happen to people standing right next to me. The head jerking is the clue. In both case (in which I assisted others to restraint the victim until medical help arrived), stiffened up very suddenly, their eyes rolled back, one spoke in a garbled voice, and then fell to the floor (both hitting their heads on desk/tables). Fortunately both recovered but at least one had not taken their anti-seizure medicine.
You are right. We need trained medical personnel to review this clip and try to analyze what happened.
Temporal Lobe Seizure