I have Quick Clean scheduled daily at 18:00
It seems to run at other times randomly and I can't work out why.
Perhaps there's a hidden schedule also.
e.g. a fixed time after last reboot.
It happens on 2 PCs XP and W7
I'm only guessing but I would imagine QC is considered low priority so if any othet tasks are running it will hold off running for a while.
I've never heard of it running unscheduled though.
You might want to open a case with Technical Support on that issue as it could be an as yet undiscovered bug.
It's free by phone or online chat and linked under Useful Links at the top of this page.