A really simple answer is that cells were destroyed or weakened and so your lungs don’t work as well as they could. So when faced with an irritation like pollen , your lungs cannot clean up like they did before you smoked.
This can improve to some degree if you stay away from the cigarettes though.

Hi DominiqueRegout , I think that the specific factors that drive or exacerbate COPD can vary from person to person .
Clearly there are some factors that can damage all of us , eg smoking & polluted atmospheric condions .
The advice I was given at the Gold Coast University Hospital is that 40% of exacerbations were from unknown sources.

While there is no known cure there is the odd interesting development .
One is a joint venture trial being conducted by the Macquarie University & the USFDA that is studying the safety & effectiveness of a procedure where those completely dead holes in our lung ( that would normally take in oxygenated air and expel CO2 ) are filled with a foam like substance that then sets. The effect is that the air breathed by the subject is not wasted in dead end holes . I believe the early results are promising with one elderly woman reprtedly gaining her feet for the first time in years .
Fingers crossed 🤞 for the success of the procedure.

Well well, searching in the med litterature... = the scientist are not understanding it yet... "Perhaps the most potentially useful area to target is to identify the specific factors that drive inflammation in COPD after smoking cessation."

Sadly it’s a progressive Disease, so stopping Smoking gives you a fighting start with the lungs but Exercise is very much needed to help clear the mucus from the Lungs so that we can breathe better, I hope this helps you
Take care
Eric