Windows Update Turned Itself On

I have always had automatic update set to notify me, but let me choose what and when to update. This morning, I got a notification that updates had been installed, and to restart my computer. When I checked, it had set itself back to update automatically. What could cause this?

I have a Dell 1545 laptop, a little over a year old. Running Avira and Comodo. All scans show no viruses or malware.

Download and run a full and updated scan with malwarebytes.
If negative, just reset updates. Probably a new update caused the change. If it happens again, post and we shall try to resolve the mystery.
It may have been caused by the install of new software or even caused by corruption when booting.
To be on the safe side, also, perform and sfc scan.
http://www.vistax64.com/tutorials/66978-system-files-sfc-command.html