Check that the drain hose is clear.
Could be it is partially obstructed or there is dust in the drip tray slowing down the water flow.

You may want to try flushing it with a 10% bleach solution. A turkey baster works when doing this. This should kill any crud growing in the drain and then flush it out. Flush some let it sit for 15 minutes and do it again. You may have to do this several times. Just be careful to not overflow the drip tray and wear old clothes unless you are into tie dye.

As a last resort you could try the following. I personally have never doe this but many others have. Take a piece of 14 or 12 gauge bare wire, wrap it once or twice around the heating element (wrapping it around a fin just above the heating coil may also do the trick and here is less worry about overheating the wire) and the direct it down the drain about an inch or so.
The idea is that when the unit goes into defrost the wire will also heat up some and clear the ice at the drain hole.
Note: You do not want it to get too hot or it can melt the plastic liner.