Windows 8.1 Reboots Computer Instead Of Shutting Down

I've got an issue with Windows 8.1 x64 where sometimes (not always), if I Shut Down the computer, it'll shut down as normal, even displaying the "Shutting Down..." message, but rather than powering the machine off, it'll reboot just as if I had initiated a Restart instead of a Shut Down.

I definitely know I haven't accidentally told the computer to Restart as it's happened more than once, I know I've selected the Shut Down button and Windows 8.1 even states it's Shutting Down and not Restarting. If I restart the computer instead of shutting it down, it'll restart just as it should, nothing wrong there.

There is no abnormal behaviour at all during this process. The computer does behaves identically to if it was supposed to Restart and Windows throws no errors, BSOD's or any trace of something going wrong and thus somehow causing the machine to reboot rather than shutdown. powercfg /lastwake gives no information at all and Event Viewer lists nothing under Critical or Errors that could be a cause.

Most of the time though, the computer will just shut down as it should, but this random rebooting is getting very irritating. If I shut down the computer again after it has restarted (when I told it to Shut Down), it will then Shut Down as it should.

Something I should note however, when I shut down the computer, I do so through Classic Shell Start Menu's Shut Down button. Every time these random reboot occur, I have shut the computer down via Classic Shell Start Menu. I rarely shut the computer down any other way, so perhaps somehow Classic Shell is to blame? Perhaps there way of telling Windows to shutdown is a bit flaky?

I believe the same issue occurred on my laptop (also running Windows 8.1 x64 with Classic Shell Start Menu) when I shut it down through Classic Shell's Start menu, but only once (I don't use my laptop nearly as much as my desktop). This problem has never occured on my desktop or laptop back when they were running Windows 8 (not 8.1).