Wet well grade change when Pump is not on

I have a pump connected to a wet well and am graphing the pump flow and wet well hydraulic grade versus time. I'm seeing that the wet well's hydraulic grade is moving up and down several times while the pump is off. I'm not sure what is causing the grade line to change when I believe the only thing controlling it is the pump that is connected to it. I can understand the grade line to increase while the pump is not running but I don't see what would make it decrease. What else could be causing this?

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