That was my take also. The mags run with the engine unless grounded, so "off" grounds-out both mgs and L / R ground only one. With the key in the "Off" position both mags should be grounded and engine stops / cannot start, but the key position does not change throughout the video (no idea whatchecks he did). So this looks like an intermittent fail in the mag grounding which allowed engine run, lift , transition and a little height before the circuit came good and failed both mags at the same time - so engine off. Possbly this fault has been around for a while, because personally if I found that the grounding of L & R in the start checks had no effect, and this was different from last flight, I would not be taking that ship anywhere - just call maintenance.
No chance of a restart at that height, so good work to get back to the ground without too much damage, and no injuries.