One another cause is referred to as "bronchitis." To understand what bronchitis is you need to understand what an "-itis" is. When "-itis" is added to a word this means the word has something to do with inflammation. Inflammation is a complicated concept. A basic way to think of it is too many kinds of cells and gunk in the wrong place. There are many kinds of inflammation - infectious inflammation (a red, swollen, hot and skin infection for example), allergic inflammation, traumatic inflammation, autoimmune inflammation, and so on.
So bronchitis refers to inflammation of the bronchi. Bronchi are medium size tubes in your lungs. See the asthma section
for a picture of some bronchi.
When you have bronchitis you have thickened, swollen bronchi that may be constricting - ie. reduction in their diameter. If you read the asthma section you can see this sounds a little like asthma. In fact an asthma attack and an episode of acute bronchitis are rather similar. So much so, if you keep having episodes of acute bronchitis year after year, you should ask yourself (and maybe your provider) "do I have a little asthma?"