Any time I ran into problems with my cup overflowing, it was because the return hose was to low in the water. I have my CSS 220 in sump and if my sump water gets to high, my return hose is then lower in the water and water will rush into the collection cup and overflow.
This is why I have my sump return pump and skimmer pump on the same power circuit, because if my sump return pump has a power failure, the backflow siphon will raise the level of my sump but my skimmer will be off as well. However, this only protects me from a power source failure, it is possible for my sump return pump to fail not because of power source issues and I would then be left my sump return pump failed and my skimmer pump running, resulting in an overflow. I have not figured a way to protect from this condition. HTH