[Alan Douglas]

He was correct; a leaky grid-coupling capacitor often causes high plate current. Whether the high current caused the break or only contributed to it, is another question.

Breaks are usually caused by corrosion where there is a defect in the wire enamel. The corrosion is either caused by acidic impurities in the paper, or electrolysis between the positive winding and the grounded core. Or if it's at the wire ends, it can be from acidic soldering flux.

Heating could expand the windings and pull a weakened spot apart.