Per Meat Plow:
Might have started out very small
and once the plastic burned enough it made more contaminants that caused
more arcing and so forth.

A vicious cycle... I like it.

But it begs a question that popped up as soon as I saw the
paddle: are some metals ok tb within a microwave oven?

I'm guessing the metal paddles are alu.
--
PeteCresswell