How do I identify the root cause of unexpected reboots?

Please try the following steps:

• Open event viewer
• expand Windows Logs
• click on system to view it
• right-click on system and select Filter Current Log
• in Event Sources: select User32
• change <All Event IDs> to 1074
• click OK

You'll now have a list of shutdown and reboot events. scroll through them and you should be able to see what process caused which reboot. The information will look like this example:

The Process "C:\Windows\system32\svchost.exe (COMPUTERNAME)" has initiated a reboot on behalf of "NT-AUTHORITY\SYSTEM" for the following reason: "Operatingsystem: Service Pack (planned)" Reason code: "0x80020010" Shutdown type: "reboot" Comment: ""

Here's a picture of the Filter to use in eventviewer, from my german system:

It seems like Windows is just as surprised as you are that it's been shut down.

That is often the symptom of a hardware failure, since most kinds of software failures would leave some sort of clue in your event log. (Normally from a BSOD)

Here's a couple things you could try:

• Try a different power bar.
  Unplug everything but the PC from that power outlet to be sure the problem is the PC itself.
• Use a power supply tester to see if the PSU is providing the correct voltages.
• If you have a spare PSU, try running the PC from that one instead.
• Run a temperature monitoring app that records temperature to a file. Some CPU models can cause an instant system shutdown/reboot if they get too hot.
• Try to run a live OS from a usb stick without storage drives and just the bare-minimum hardware connected. If the problem still occurs then you at least narrowed it down to your CPU, motherboard, RAM, or PSU.
• Run a CPU & GPU intensive stress test to see if you can reproduce the shutdown quicker. (Will help you narrow down the problem & save time troubleshooting.)