I'm not a medical professional, but I've seen breakdowns of similar wounds. I think this can be compared to being shot in the heart. It creates a massive blood pressure increase that my guess in this case causes the brain to react with fencing. I've read that the pressure spike can be the cause of death before blood even leaves the body. Either that or some side effect of damaging the brain stem.

The more I watch it the more I think it was instant lights out. Like the spinal cord was probably cut as well as the carotid. Never heard of fencing response but I just read up and yea I think you might be right.