What disorder are you talking about, specifically, because every personality disorder has different causes and "avoidance disorders" is not a specific mental illness category nor a specific mental illness, unless, you're asking about "Avoidant Personality Disoder" (APD) and I think you're asking about anxiety disorders, in general, which have different causes. The National Mental Health Association defines "Avoidant Personality Disorder," which is also an anxiety disorder as follows: "Avoidant Personality Disorder Avoidant personalities are often hypersensitive to rejection and are unwilling to become involved with others unless they are sure of being liked. Excessive social discomfort, timidity, fear of criticism, avoidance of social or work activities that involve interpersonal contact are characteristic of the avoidant personality. They are fearful of saying something considered foolish by others; worry they will blush or cry in front of others; and are very hurt by any disapproval by others. People with avoidant personality disorder may have no close relationships outside of their family circle, although they would like to, and are upset at their inability to relate well to others." source: http://www.nmha.org/infoctr/factsheets/91.cfm If you'll rephrase your question, then I can answer it more clearly because different anxiety disorders like what you're talking about have different etiologies (i.e., origins). :)

If there is some activity or situation (such as socializing) that causes you to feel negatively, you may choose (consciously or otherwise) to avoid the situation. This avoidance provides a relief from the negative feelings, so the act of avoiding becomes more and more reinforced the more it's repeated.