Anti-Virus software was scanning the volumes, thus updating the 'ctime' on the folders, warranting a backup of them.
Disabling an aggressive AV scan on these volumes is one approach. In some cases, changing the AV software to allow a less aggressive scan will not update the volume info when this happens.
Also, on Linux/Unix systems, the File System plugin checks for the 'ctime' on volumes during an Incremental backup. Thus, you will want to make sure that the option for 'Reset Access Times' is checked if the AV software is changing the 'ctime' so instead NV looks for the 'mtime' or modified time instead.