Section 212 of the immigration Act specifies 10 reasons for refusals, including:
Health related, Criminal records, Security grounds, Public Charge, Labor qualifications, Illegal entrants and immigration violators, Documentation requirements, "Ineligible for citizenship"!, Aliens previously removed, miscellaneous (polygamists, unlawful voters, etc..)
I believe that visa officers are required to clearly specify the reasons for rejecting any application, base their decision on one of the above reasons and give chance to the applicant to submit any supporting documents.
However, this is not always the case, as they sometimes submit the refusal document after finishing and leaving the interview, with no chance of further explanation, defense or document submission.
Furthermore, officers may transfer any case for additional 'Administrative Processing' allowing for cases to expire before the visa deadline.