The back of the mouth drips fluid and runs down into the respiratory tract, starting a coughing attack, or into the voice box.
Most people only experience postnasal drip when they are sick with a bad cold or something similar.
But some have it when they are well. This article is for them. Normally, these secretions, from the sinuses, flow down the back of the nose and throat and are swept away by cilia, which are small waving hairlike projections. But sometimes the mucous dries out and the cilia no longer wiggle. Then the secretions pool in the back of the nose, thicken, and then begin dripping into the bronchial tubes or into the voice box.