Visit family sponsor visa rejected…security reasons ??

Hi guys I have a question on how to go about this or what this means exactly. A relative got their visa rejected and it was stated for security reasons and no other information was disclosed.
Now for background the person had left the country a while back for a funeral during the time away their visa had expired as they couldn’t return in time for the renewal due to some problems in their home country they faced. Now while they were away they weee notified that they have a fine to pay off from some property dispute and can only then return as they didn’t address the property issue ( since away). They paid off the fine and planned for the return. Now just to make sure all is good despite having nationality from North America they took out a family sponsored visa rather than visa on arrival to make sure before booking any flight that no issues will arise. But the visa was rejected and stated security issues ? How can one know the exact reason? And if immigration has the person flagged in the system how can it be addressed and sent the correct documentation to show all has been cleared ? Thank you