If anything it would be stimulus-sensitive myoclonus. It is triggered by a variety of external events, including noise, movement, and light. Surprise may increase the sensitivity of the individual.
The reporter obviously was the “trigger” to Hilly’s head jerking reaction ..... it certainly surprised the reporter who cringed back & looked a bit horrified ... that reaction leads me to think something other than “joking around” was going on with Hilly, whatever it was/is.