Strange, I have no idea how that's possible either to be honest.
My old X58 motherboard did that all the time after ~2 years of heavy use, not sure exactly when it started doing it. I could shut down via windows or the power button, and it would auto-restart after 5 seconds. Never figured out why, but I ruled out the wake-on events, shorts, and the case itself. I'm so glad to be rid of that motherboard now