In this case it's air getting in the way of the water. There's a vacuum created when water leaves the container, but the water is heavy enough to still fall out. The air displaced by the water is light enough to travel to fill that vacuum, but if the mouth is narrow enough, the water gets in the way until the forces are sufficient for a bubble of air to travel into the container, interrupting the flow until the forces are such that water flows again.
What do you think primarily influences the sound when water is poured from a bottle? Is it the material of the bottle or something else?
In this case it's air getting in the way of the water. There's a vacuum created when water leaves the container, but the water is heavy enough to still fall out. The air displaced by the water is light enough to travel to fill that vacuum, but if the mouth is narrow enough, the water gets in the way until the forces are sufficient for a bubble of air to travel into the container, interrupting the flow until the forces are such that water flows again.