I have a theory; (don't shoot me , its just a theory

)
I think its not isolation that leaked, but just some grease
many time metal has a very thin film of oil or grease to prevent rust.
now the ptansformer is build out of many layers, and they all have this protection from the factory
so i think during years of use and thus years of heat the oil has "melted" and made its way to the bottom
there it collected on that spot into the wood, and it came loose when you removed it from the wood.
does that make any sence

don't know, just thinking here
