Evaporator Fan Freezes up

When I removed the panel between the freezer and fridge (freezer on bottom) the evaporator fan is found encased in ice.

The drain line is frozen where it goes through the back case and seems to have backed up the water where it freezes and eventually encased the fan.

When thawed the drain line is free of obstructions. This has happened several times in the last two months. What could cause this drain to freeze.