Well, if I could answer that, I could be a billionaire!
Keep in mind that each of us is different, but here is my short story.
My diagnosis is asthma, bronchiectasis and hyper reactive airways, and here's what causes exacerbations for me - stopping airway clearance, viral respiratory infections like a cold, getting extremely stressed and overtired, not managing my asthma & allergies, being exposed to smoke (like this summer's fires) and some chemicals. I will get inflamed airways, bronchitis, pneumonia or even pseudomonas (I think that is always lurking in my lungs, along with MAC, but grows much faster.)
If I catch it immediately & treat with extra levalbuterol & saline nebs, extra rest and lots of good fresh fruit & vegetables, I can sometimes recover spontaneously. If not, it is on to steroids and antibiotics.

Can you figure out your triggers based on your most recent exacerbation?
Sue