Just had the heads replaced, what could have cause them to blow on 1997 Ford Escort

I had the heads replaced about 4500 miles ago. What could have caused them to blow?

Asked by for the 1997 Ford Escort
Ford Escort cylinder heads tend to crack, if the engine ever gets too hot. So, you must keep the radiator working properly, the coolant fresh, the cooling fans working very well and the thermostat replaced every 60 k so that it does not happen again.