I've seen some threads regarding this issue but with no solution (apparently) so I decided to add my case to the stack.I have a newly bought NUC D54250WYKH, installed with a 240GB SSD 2,5" SATA drive (don't know if it is relevant), with Windows 8.1 as Host SO.
I have letting my NUC powered on all the time because it is running a Windows Server as a VMware Workstation guest, but today I've discovered that if I shut it down, after a while (can be seconds or several minutes, without any pattern), it powers on automatically.
I have Wake On Lan (WOL) ready to be used because I might need to turn it on when I'm not at home, so I had some days ago, to disable "Fast Startup" on Windows because if it is enabled, the Network Interface Card does not "listen" any magic packet when you "normally" shut down Windows, only when you do a SHIFT+Shut Down, so I disabled this feature in Windows 8.1.
I have also tried to disconnect all USB peripherals (USB 3.0 Toshiba HDD and Rii remote keyboard), and take batteries out from the Logitech Harmony remote control (just in case...), without success.
I have latest BIOS WYLPT10H.86A.0033.2014.1201.0940.