The most common cause would be air buildup in your filter. Air can get in there, either from taking the equipment apart for maintenance or through a suction side air leak. Air will build up in the filter all the while getting compressed as the pump is running. When the pump shuts off that air expands forcing the water back through your line, up through the skimmer and that’s where the lid pops off.
If you have return jets controlled via valves, and those valves are partially closed, not as much water can be allowed through to the return. Backpressure builds up and when the pump shuts off, the path of least resistance leads the water to flush back to your skimmer to blow the lid off.
Another thing we’ve heard from a few homeowners: if the filter is very dirty and it needs to be backwashd, the water will create back pressure and when the pump turns off it will rush back into the pool. I’ve never seen it happen myself. Their lives could be as dirty as their filter, but if it happens to you that might be one of the things causing it.
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