The triggers of Asthma depend solely on the person that has is. Triggers can be anything from physical triggers like peanuts, mold, or grass or psychosomatic triggers like anxiety or distress.
In order to know the triggers you have to what asthma is:
"Definition
Today asthma is viewed as a chronic (long-lasting) inflammatory disease of the airways. In those susceptible to asthma, this inflammation causes the airways to narrow periodically. This, in turn, produces http://www.answers.com/topic/wheezing-3 and breathlessness, sometimes to the point where the patient gasps for air. Obstruction to air flow either stops spontaneously or responds to a wide range of treatments, but continuing inflammation makes the airways hyper-responsive to stimuli such as cold air, http://www.answers.com/topic/exercise, dust mites, pollutants in the air, and even http://www.answers.com/topic/stress and http://www.answers.com/topic/anxiety." - From Answers.com