Possibly because I finally understood and found my way to an airway clearance postural drainage position(s) that I could do and felt safe doing. Also possibly because I improved my understanding of the deep breathing methods needed to help get air behind the mucus plugs to help get them up and out before doing the huff coughing. Possibly because I drink a great deal of water to loosen the mucus, possibly because I boil a great deal of water and try to keep the house from causing too much dry air. Possibly because I do self "percussion" on my lungs, front and back as often as I can and especially before the final morning routines leading up to the air way clearance of huff coughing. Possibly because I try to eat healthy to keep my immune system up. Possibly more things than I realize. Who knows.
Hard to know exactly but like in many things in life it is never just one thing it is usually several things that bring about change and all working together.