These gamma rays are associated with weather that produces lightning, but the actual mechanism for gamma ray production by weather was still uncertain at the time.
The video explains that a discharge mechanism called "Dark Lightning" might produce the gamma rays without producing the same visible light signatures that normal lightning produces.
Question: Did Dark Lightning turn out to be a real thing? Is this what produces gamma ray flashes from lightning storms?
Screen shot from the video shows a graphic of a booklet with the following words on its cover:
AGU Fall Meeting, San Francisco, 3-7 December 2012