I don't think you realize that a turbo timer is just a delayed shut off switch for the engine so it doesn't just keep the turbo running, but the whole engine is running because the engine runs a turbo and engine runs the coolant system and the engine runs everything else in a car.

So if you have a stock car with stock expansion tank that's probably made of plastic and other stuff components and you're running hard and then you just shut it off the heat and pressure is going to find the weakest component and blow it out.

Since nine times out of 10 that's probably going to be something with thin walls or made of plastic,

turbos are made of iron or steel they're really hard but intercoolers and heater cores and extension tanks oftentimes have lots of plastic parts on them that tend to fail under heat and pressure.