Usually, it is because of some missing information; sometimes it is audit.
Nowadays, usually there are no interviews excepting in one or two cases INS likes to audit at random. Some cases where interviews were held for other reasons have appeared earlier on this website. In one case, the person did not have his name on his birth certificate and hence he submitted two affidavits from parents. Possibly because that person was born after 1970, INS wanted a non-availability certificate from the municipality in the place of his birth. In yet another case, INS wanted some missing information in a person\'s biodata (as far as I remember, it was about that person\'s status in a particular period).