I heard different. RUMINT from my SW peeps I heard was that this was a newhire FO (recent AF retiree, wasn't even done with consolidation) attempting the landing. He misjudged the landing height and rounded out high (it's clear by the video btw). More than couple potatoes went by and so he called his own go-around. That's when Captain-
"we don't go-around @ the airlines" overrode his call, took control of the aircraft and nosed that guppy over like she was gonna miss her commute flight home.
The stunt was done too late in the botched high roundout, no airspeed left; the nose collapse all but writes itself at that point. It was my understanding it wasn't the nose-over that did her in, but the fact she dismissed the FO go-around call to add insult to injury. She also had a record of being combative/dismissive with FOs, so a bit of peer review went into that head shot.