The liquid tries to pass in to fill the vacuum.
Can there be a vacuum in contact with a gas or a liquid?
No; because the gas or the liquid will instantly fill the vacant space.
Why does water make a gurgling noise when poured out of a bottle?
Because air is passing into the bottle at the same time that the water is coming out, and the air makes the noise in passing through the water.
Why does water make a gurgling noise when running out of a barrel?
Because the air passes into the barrel at the opening where the water passes out, and thus the noise is made.
How can the gurgling noise be prevented?
By making an opening at the top of the barrel, so as to admit the air.
Does water flow freely from a vessel that has but one opening?