This article explains in detail how to find the reason for last startup/shutdown. In my case, this was due to windows SCCM pushing updates even though I had it disabled locally. Visit the article for full details with pictures. For reference, here are the steps copy/pasted from the website:
Press the Windows + R keys to open the Run dialog, type
eventvwr.msc, and press Enter.If prompted by UAC, then click/tap on Yes (Windows 7/8) or Continue (Vista).
In the left pane of Event Viewer, double click/tap on Windows Logs to expand it, click on System to select it, then right click on System, and click/tap on Filter Current Log.
Do either step 5 or 6 below for what shutdown events you would like to see.
To See the Dates and Times of All User Shut Downs of the Computer
A) In Event sources, click/tap on the drop down arrow and check the
USER32box.B) In the All Event IDs field, type
1074, then click/tap on OK.C) This will give you a list of power off (shutdown) and restart Shutdown Type of events at the top of the middle pane in Event Viewer.
D) You can scroll through these listed events to find the events with power off as the Shutdown Type. You will notice the date and time, and what user was responsible for shutting down the computer per power off event listed.
E) Go to step 7.
To See the Dates and Times of All Unexpected Shut Downs of the Computer
A) In the All Event IDs field, type
6008, then click/tap on OK.B) This will give you a list of unexpected shutdown events at the top of the middle pane in Event Viewer. You can scroll through these listed events to see the date and time of each one.