Why did the wires burn on the transformer TMO?

And here we are talking, if I understand correctly, about a transformer for heating concrete. Cables are burned for one reason - increased heat. The reasons may be different heating:

1. Too much current. Take a cable with wires of a larger cross section.
2. Short circuit - actually at this moment also a very high current.
3. Incorrect installation, for example, when a cable is used rolled up into a ring or a bunch of cables are piled up, all are loaded, heated and do not cool.
4. Poorly tightened pins. They are warming, warming and veins as a result of isolation peeling off.