the specific gravity of the water. your pumps inlet, to have a good suction, is on the bottom part of the tank. the specific gravity of cold water is denser then the air warmed surface of the water. the warmer water is being pushed away with the denser cold water from the bottom with the pump. to stop this try to heat the water by a long black hose in the sun light like a solar panel off the exit of the pump before it gets back to the tank.
This normally happens when cold water comes up in water tank during summer when the motor pump water pressure gauge oscillates from low to high pressure as it pumps it to the tank. The pump runs continuously for the water supply system to be effect.