Apparently, the company became a target after the hacktivists learned that it offered its services to cybercriminals and fraudsters.
“Many websites use WHMCS to scam and rip people off. For example: Users from "hackforums.net" are using WHMCS to sell illegal hosting, booters, malware, etc,” a member of UGNazi explained.
“We have reported these sites to WHMCS before and they did not take any action whatsoever to stop the illegal activity. By releasing their files, we wanted to make it known that we are watching; and will continue to be watching.”