What causes someone to be introverted?

There are multiple: possible causes of introversion. It may be hereditary, learned, due to self consciousness about physical attributes, lack of self confidence.

Many possibilities: There are a number of theories. however, the literature seems to lean toward an inborn trait that environment has caused to be rather fixed. This can be changed with psychotherapy if one wishes to.

Many things: Some people are characterological he predisposed to being introverted. This means and cleaning which they are that way by nature, and they have had no aversive experiences in crowds to make them introverted. Others are introverted because of having poor experiences in dealing with crowds which can include fear of judgment, being put on the spot, etc.It depends on each person.