Your evaporator coils frost up in normal use and every eight hours or so the entire unit shuts down and the defrost heater comes on to melt the frost. This cycle last about 20 minutes. The melted frost drips into a drain pan and through a drain tube to the drain tray under the freezer/refrigerator where it's evaporated by the condenser fan.


Your drain tube may be stopped up with ice at the upper end because it drains too slow because it's stopped up at the lower end in the evaporator pan under the unit at the floor. It can get dust and mold in it. Once you get the ice out at the top a little pressure with a turkey baster will usually clear it out. Flushing it out with hot water and clorox may help.
Make sure it drains quick enough to prevent refreezing. . The drain should be located below the evaporator coils on the lower back of the freezer. If this repeatedly happens with yours, you may can prevent it by taking a bare copper wire and wrap it around a fin near the heater and put the other end a few inches down the drain. The heat convection may prevent it from freezing.