It used to be 737 post de-icing procedure to cycle the ailerons and elevators to full deflection in both directions in the before take off checks to drain de-icing fluid out of the balance cavities until just a few years ago, when the procedure was removed because the drain holes were drilled larger. I wonder if that old practice also had an unknown benefit of moving any slush off the control mechanisms, so masking the issue and it only coming to light recently. I still cycle the controls three times anyway - it might be un-necessary, but it costs me nothing.