Hang-on skimmer is overflowing question.

[ReefOcean]

I woke up to discover my skimmer spewing waste from the collection cup back into the chamber (essentially into my tank). My skimmer will randomly create very wet skimmate, and cause the collection cup to fill up. Before, I thought it was a water level issue (water level changing from evaporation) but now that I am using a sump, the water level int he diplay is always at the same level. The skimmer will work good for sometimes months, and then all of a sudden it will overflow the cup...

What could be causing this?