Most commonly sand from the engine casting process, and other debris settles harmlessly in the heater core. If the coolant isn't replaced every two years, it is normal for acids to build up in it, and that leads to corrosion of metal parts, most notably the heater core. That sand and debris often blocks the leak. When any engine work is done that requires replacing the coolant or flushing the cooling system, that sand can be washed out exposing the leak that was already there. This happens most often to conscientious mechanics who are trying to do the best job possible for you. They didn't CAUSE the leak. Their work simply uncovered the leak.