Updated to include news article.
Humans holding on to their tools is not something new, we see it in a lot of instant high stress situations. We see the same thing often in police dash-cam video where the officer clinches his or her clipboard or other items they are holding. This is the first video I have found containing what looks to be an untrained individual experience the same potentially involuntary response.
It’s because of this and numerous other factors that may result in only one hand being available, that I choose to train myself and my students to not only shoot, but to complete every problem solving action common to a handgun with one hand.
The news article covering the event can be seen here.