One of two speculative scenarios comes to mind:
- You added a cron(tab) entry to prevent locking yourself out by powering off sys-usb
- The Qubes installer adds that nowadays if it detects an installation with a usb keyboard
In any case, seems to be “by design” and it has to be an entry in one of the many /etc/cron* , /etc/anacron* , and /var/spool/cron/* files and directories.
EDIT: in dom0, of course.
EDIT2: upon reading your message again, I’m going to wager a small amount of virtual karma that you have an entry in the “/var/spool/cron/user” crontab file 
EDIT3: … a file which you manipulate using “crontab -e” as user “user” in dom0. Now the answer is complete!