it's normal. You can short cut the process by opening the air relief at the top of your sand filter, but it really is nothing to worry about.
The cause will be the level of the top of the sand filter or pool pump against the level of the pool water. Don't worry about it.
Sorry should have said thge air relief is normally at the top of the sand filter, behind the pressure gauge. It's a turn screw knob with a hole half way along it's length that bleeds the air from the sand filer/pump.
You undo it until you hear air escaping, thewn sut it when water comes out of the hole instead of air. Don;t undo it too far or when the water starts to flow it kight be a pig to get back in