Well this one's easy. I know it's kind of stating the obvious, but...
Flying along kind of low, brought the a/c to an OGE hover. Started settling vertically, maybe even a little backward, had lots of power pulled. Let's see, what happens when we do a vertical descent with power pulled? What's that condition called again? I forget...
Anyway, he must've kept pulling because the low-rotor horn sounds and continues to impact. Manifold pressure gauge is up at 29", tach needles are pointing straight at each other. Not good! He must've tried to fly out of it because at :14 we see he's got the nose down 12 degrees or so and has started to get some airspeed back. Too little, too late...crunch.
Textbook settling with power. Or, as Nick likes to say, settling with insufficient power. If only Frankie had put a dang turbo on that 540! If only...