Snippet from the article written by Dr. Maria Hordinsky, a dermatologist from the University of Minnesota:
Stress has been cited frequently as a factor in autoimmune diseases, including alopecia areata. Yet experimental evidence to decisively link stress and alopecia has been lacking.
It is easy to link alopecia with stress, as many but not all patients associate stressful events with the development of hair loss. Moreover, there are a lot of nerves in and around hair follicles. The actions of these nerves, as well as various nervous system products called neuropeptides, could affect the hair growth cycle.
Read the full text at NY Times — Can Stress Cause Hair Loss?
I have seen a few cases of surgical stress induced alopecia areata. One patient that comes to mind had a hair transplant surgery just before the alopecia areata appeared and it was active for about 6 months.