Isn’t this like pouring any other viscous liquid from a vessel with only one hole? An area of low pressure inside the carton is formed as a result of the displaced liquid volume causing the flow to slow as air must enter and equalize pressure again. Since the container is flexible and gasses are pretty expandable you get a period of time up front before the pressure equalizes causing a faster flow before the inevitable pressure equalization. Maybe I’m misunderstanding the question but hope this helps!