tl;dr: Non-spec DisplayPort cables were feeding back power from the monitor to the video card, causing my computer to reboot during POST. Buy this cable. //

When I got down on the floor to look inside the case, I saw something moving.

The case fan was spinning.

The freaking case fan was spinning. And I was holding the unplugged power cable in my hand. How is this even possible?!

I unplugged all of the PSU cords from my motherboard, and things shut down. The GPU light turned off, and the fan died.

The next day, I returned to Google, and for the hell of it, searched “computer is on even when unplugged.” I know, it sounds ridiculous, but it lead me to the answer I had been looking for (in addition to a funny meme of The Pope casting holy water for someone with a similar issue).

I scrolled, and scrolled, pages and pages and URL after URL of articles on something called The DisplayPort Pin 20 problem started showing up. It felt so funny to search for months with no solution and then to almost be assaulted by the number of posts on the same issue I was having.

It turns out that cable manufacturers who don’t adhere directly to the official DisplayPort spec end up connecting the 20th pin in the cable on both sides. That pin, carries – you guessed it – power!

This issue was serious. Enough power had been backflowing from my monitors into my GPU to run my case fans when the system was off.