I did not know there were only three of them.
1. Computer Error (could include other errors)
2. Applicant LYING
3. Applicant having an UNREASONABLE amount of outstanding debt (this value is debatable)
Actually, I think there were 4 now, IIRC an applicant who has had a history of illegal activity can also be denied, especially when having a history of fraud. But to my knowledge, at least in North America they are not allowed to do background checks on criminal reccords, at least not without your permission.