It really depends on the size of the leak. I once did headgaskets on a friends gm 3.4 v6. That thing would be viciously boiling within 5 minutes of starting cold. When I had the head cracked on my 2.6, it would only boil if it had bubbled long enough and run the coolant low.
Sometimes the air pockets will vent out of the cap easier than expanding coolant. If the cap is less than perfect it will vent the pressurized air much more quickly than coolant, and sometimes below the pressure rating on the cap. When this happens the pressure in the cooling system drops, lowering the boiling point of the coolant, and sudden violent boiling follows