If the freezer door is left ajar the cooling coil at the top of the freezer behind a grill becomes a solid lump of ice. The freezer compartment then doesn't get down to the correct temperature and runs continuously.
Air is circulated over the cooling coil by a fan, if the coling coil is covered in a solid lump of ice the air doesn't circulate and the freezer doesn't cool.
kandlbarrett, October 2009