Glupping air helps in digestion. Since the knives have such a short digestive tract, the air facilitates in digestive processing.
When my knives were babies they would calmly swim to the top to glup the air, but then one of them started making a big deal of it by splashing at the top. Soon all of them started to do it. They are never quiet and calm anymore. I think it is cool, but when I have people over they are a bit distracted by the splashing noises