Simple transformer insulation failure? It only takes one turn to short out and rapid overheating follows causing progressively more insulation failure. Possible causes? Weak insulation since manufacture, moisture ingress, vibration, transient voltage surge, hotter than designed operating environment, - I'm sure there are more I can't just think of.
100% agree with that Greg> I remember a plant where some employees used a 75KVA for a lunch warmer with help from some cardboard & welding screen curtains! A situation that could have become a njghtmare.
We had an old Westinghouse padmount 12kv pri 480V sec 1000kva that inadvertently was loaded up with >1600 hp in oil pumping units & steam genset pumps & blowers.. It ran this way for quite some time & wasn't discovered until we ran into some low voltage issues in a control room... The load was divided up with 2 banks of 167s added. The padmount still runs part of the field..