To go to the first 2 easiest things I can think of is
1. believe it or not people are allergic to cigarettes and alcohol. (the tobacco in the packs have over 4000 chemicals) You could be allergic to one or a few of those chemicals causing your body to instinctively produce mucous in your nasal canal to help protect the your nasal membrane.
2. The smoke could be drying out your nasal canal and your body realizes that and is producing more mucous to keep it moist.
3.You may have a cerebral fluid coming down from your brain. The smoke may trigger it.
The best way to find the answer is to stop smoking or just go straight to your EMT doctor and ask .