When drinking from an inverted bottle, a vacuum starts to appear at the top as water flows out of the bottle. Because of the air pressure outside, air forces its way through the neck of the bottle and bubbles up. This is followed by more water escaping, and more air bubbles moving up. So on and so forth. The glug-glug is caused by these two alternating processes.
The fastest way is to pour water at an angle and with a swirl. To create a swirl, move the bottle in small circles before pouring. This way, water moves to the side of the bottle and no gurgling occurs, allowing air to freely enter through the centre.