This can happen from a few things, Windows updates, some application that was pushed to the system by an administrator, someone that has access to your computer sending a remote shutdown command, shutdown command setup in a scheduler. Is this a work computer, home computer? Check scheduler to see if there is a shutdown command set to run. Does this happen all the time at the same time? Does it come up soon as you start the computer? Does it come up after the same amount of time?