I've run into the same thing. One thing to consider is the 2 main ingredients of e-juice. Pg is commonly used in breathing treatments. These breathing treatments, while administering meds, are also used to get the patient help expel irritants by causing mucus production as to collect irritants to more easily caught them up. May not be the most correct answer, it just some of what I picked up while leaning all about my mothers breathing treatments following her lung transplant (not smoking related).
as for the VG, it naturally absorbs moisture which is why you can get dehydrated. The moisture it absorbs can also add to the added mucus. Although I'm now not coughing near as much as I did with analogs, I've found when I do cough out what remaining crap is in my lungs it tastes like what ever flavor I've been vaping. Sorry, gross I know, but had to share for insight.