In the early medical reports the cause of agoraphobia (AG) was described as a lack of self-confidence spurred by the sight of large grounds or extensive structures, which essentially represent the spatial organization of big cities. According to that model, the prominent feature of agoraphobia is a fear of self that is rooted in the groundless fear that the self will not be able to fulfill certain duties.
For example, individuals with agoraphobia would fear not being able to walk a certain distance, or go without food for a certain time. In other words, agoraphobia would be aptly called autophobia, or “fear of the self”.
Concerning the etiology (i.e., cause) of the disorder, it is believed to affect otherwise healthy persons with an inclination towards excess and overindulgence (e.g., alcohol, foods, and even sex), and thus it is argued that agoraphobia is more a symptom of the larger disorder that autophobia is.
Reference:
Sutherland, H. (1877). On Agoraphobia. Journal of psychological medicine and mental pathology (pt 2), 265-269.