I have been told by my doctors and wish I had more specific information but there is a nerve in your throat that reacts to irritation and can overreact causing massive amounts of mucus and phlegm.

I have what others are posting here about this. With knowing the cause (at least for me) I know it is coming and prepare for it. Mine happens ever time I eat. I found soft things like mashed potatoes don't cause it. But I have learned to drink water when I eat to help keep food down and not irritate this nerve.

I am told the nerve not only creates mucus in throat but also sinuses thus the drainage from sinuses back down the throat mixing it with the mucus being made in throat by nerve stimulation.