I doubt that “charge buildup” is part of it – there’s nothing in the beet that would rectify. However, iron (being ferromagnetic) soaks up a huge amount of energy from rapidly alternating magnetic fields (microwaves, no?) and would produce hotspots. Quite possibly steam, which in turn would be conductive enough to sustain short arcs in the intense electric fields.
Fun question.
As for Popeye, his original wonderfood was … garlic. Sponsors got it switched to spinach for the cartoons.
