Do you think earth bonding one side of the mains transformer's secondary might have helped?
The secondary side was probably floating at 60VAC or more and bonding it would reduce the maximum voltage relative to earth to just 24VAC. This means that the tip voltage will be even lower if this happens again.
Even better, if it's a centre tapped transformer, earth the middle so the maximum voltage with respect to earth is just 12VAC RMS or 17V peak which is low enough not to fry a MOSFET gate.