maybe it's the operating system itself doing a repair or maybe, if you have auto updates running, windows downloaded updates and they need to install.
maybe you have a lot of crap running that you shouldn't. when is the last time you looked in the msconfig box to see what is running and what is running you din't need running?
what services are running that shouldn't be or you don't need?
when is the last time you seeked out virus' or cookies or malware? all of those will keep your machine from shutting down right.
when is the last time you looked in the firewall to see what's running you don't need?
what programs do you have running in the back ground that either auto update or stay open on a constant?