Cause 1
Clogged or Freezing Defrost Drain
The defrost drain might be clogged or frozen. If the defrost drain is frozen, the water will overflow the drain trough and drip down to the bottom of the compartment. If the defrost drain is frozen, thaw it out and ensure that the drain is clear of debris.
Cause 2
Drain Strap
Some defrost drains are outfitted with a drain strap. The drain strap is a small piece of copper or aluminum that attaches to the defrost heater and extends partway into the defrost drain. When the defrost heater turns on, heat is conducted through the drain strap to melt away any ice that may have accumulated further down in the defrost drain. If the drain strap is not in place, the defrost drain may be more likely to freeze over.
Cause 3
Drain Heater
Some refrigerators are equipped with drain heaters. Drain heaters prevent the water in the drain pan from re-freezing and clogging the defrost drain. If your drain heater is defective or is not in the proper position, the defrost drain might freeze over.