Placing the HVAC on fresh air, and increasing the fan speed, raises the air pressure inside the cabin, forcing the water down the drains.
HVAC on recirc decreases cabin air pressure, relative to the pressure of the air pushing against the bottom of the sunroof drains. Thereby causing water to drawn like a straw gurgling up through the hoses.
Many years ago (maybe 20 or more) there was a TSB regarding dust being drawn up around door seals due to the negative cabin air pressure relative to that of the air rushing around the exterior of the moving vehicle. I can’t quite recall, but it may have been for the Gen 1 1999-2000 Escalade, that introduced triple door seals on the GMT-400 platform.