The exact cause of alopecia areata is not known. It is understood to be an autoimmune condition. Research evidence is limited, however scientists believe it is likely that there are a number of causes/triggers. It is unlikely to be the same trigger/cause for everyone. These triggers could be something from inside the body or something outside the body, or a combination of both. 

Stress is a commonly quoted cause of alopecia areata but there are lots of people who cannot link their alopecia areata to a stressful event or period of stress. This article from researcher Professor Kevin McElwee provides a comprehensive look at the possible causes of alopecia areata and is an interesting read for anyone wanting to understand more about potential triggers.