What causes COPD flare-ups?

I have been diagnosed with severe copd. I have been in the hospital x 3 since April. I am going to pulmonary reh. x 2 a week. I would like to know what causes these flare ups? is it doing too much, having s.o.b. and regrouping is it the humidity outside? I tend to be a very busy person and doing things quickly, If anyone has some types on these flare-ups I would love to hear from you.