Plugged defrost drain line in KitchenAid...

Question

We have had this unit in continuous operation since 2000 and recently encountered water dripping into the refrig compartment from the air vents just below the freezer. Removed the freezer drawer and assembly and found the freezer floor covered with ice and the drain line at the back of the unit, frozen. Removed the ice and melted the plugged line (using a hair drier and boiling wate) Line now appears clear. I assume something plugged the drain and it eventually froze? How did something get into the line or is their some other issue related to the line getting plugged? Thanks, Fred