Reasons for visa blacklisting in the UAE?

Hi all, I am new to this forum and found it very useful. One of my friends got a job offer from a reputed company in Al Ain, UAE and HR applied for the Visa. After a few days the HR informed him that the Visa has been rejected by Immigration due to blacklisting. He was shocked as he had already resigned from his job with his current company in India. HR said they will try again.

What might be the reasons for getting rejected? He has never come to UAE nor does he have any bad reports or cases back home, nor bad loans or credit cards. What are the chances of getting the Visa approved after applying a second time? How can he rectify this issue and be removed from the black list? Please give your valuable feed back as the guy is very stressed as he is sole breadwinner for the family. Thanks