Many people i know with copd have more trouble with their breathing, when they quit smoking. why is that?

Lungs start to work: one of the side effects of smoking is poor clearance of mucus from the respiratory system. Many long time smokers who quit (the right thing to do) will get a lot of coughing as their lungs wake up and get rid of all that junk that has been sitting there for ages... It gets better and the lungs can recover from the smoking with time.