The law in the United States mandates an interview be performed by a consular officer before a visa application is approved. These interviews are performed at a U.S. consulate or embassy and -- depending on the information revealed in the interview and how it applies to U.S law -- the application will either be denied or approved.
Most visa applications get approved, but there are a lot of scenarios and circumstances that could result in the denial of an application. For example, applications get denied with consular officers are unable to look at all the information they require. They might be denied when applicants fail to qualify for the particular category of visa that the applicants applied to. Information pertaining to current or past criminal activity could also be grounds for an application's denial.
Once the denial happens, the applicant will receive notification of the denial. He or she will also receive information about the legal reasoning for the denial. In some cases, denied applicants who have been ruled "ineligible" will be informed that they can apply to waive their ineligibility status. These waivers are granted by the Department of Homeland Security. Unfortunately, some ineligibilities are permanent and cannot be overcome. A list of ineligibilities can be found in the U.S. Immigration and Nationality Act, and in other laws.
Canada residents who have been denied entry to the United States may wish to discuss the matter with an experienced immigration lawyer. Just because one has been denied a visa does not mean it will be impossible to overcome the hurdle. Numerous legal strategies exist, which can be employed on behalf of U.S. visa applicants. In fact, the law is not always perfectly clear to the uninitiated and a lawyer may have a lot of ideas that the visa applicant was not aware of.
Source: U.S. Department of State, "Visa Denials," accessed July 01, 2016