There are no specific reasons for social anxiety disorder but it may be caused due to a combination of reasons like genetics or surrounding environment.
A person who might have experienced neglect and abuse or whose close relative suffers from social anxiety disorder has higher risk of suffering from the disorder.
According to the Diagnostic and Statistical Manual of Mental Disorders, social anxiety is the phenomenon in which a person behaves differently due to the fear of being judged and the anxiety affects their normal routine as well as their relationships with others.
Moreover, in people who have social anxiety disorder, the feeling of anxiety remains persistent for about 6 months in these people.
Social anxiety causes distress in people when in a specific social environment, in a situation that requires meeting new people or having a small talk.
They might also experience performance pressure before giving a presentation or feel anxious before raising a toast in a wedding.
The social anxiety disorder is generally ego dystonic, meaning they feel more anxious because of the fear of people knowing about their anxiousness.
They might be afraid that the physical symptoms like trembling and blushing shall be noticed and judged by others.
Severe anxiety due to the disorder may cause derealization in which a person spaces out and is unable to recognise their surroundings.
People might take help of drugs and alcohol to get rid of their anxiety that may lead to the problem of addiction.
Source: https://fit.thequint.com/mind-it/social-anxiety-disorder-causes-symptoms-diagnosis-and-treatment