What we do know is that there are certain triggers that seem to be fairly consistent for asthma. One is allergens. The majority of people who have asthma also have some allergy to an airborne substance. However, around 20% of people do not, so the two aren't necessarily linked. For others, food will cause an asthmatic reaction, which also sounds a lot like an allergy problem. Smoking and some medications can also cause similar reactions and triggers for asthma.
However, there are non-substance based triggers. For example, exercise can induce an asthma attack, or even simply having heartburn. On top of this, you can find weather and emotions also causing the asthma to hit.
For most sufferers of asthma, it's essential that they keep a record of what has happened when an asthma attack occurs. Because of the enigmatic nature of asthma, it's up to each patient to figure out what his/her triggers are and maintain them on a personal level.