Why does my Windows computer immediately turn back on after sleep/hibernate?

To continue from Nicholaz's answer this reveals some wake events in the event log that I missed out, since they were not errors:

To find out what caused your computer to wake from sleep, do the following:

Open Event Viewer by clicking the Start button , clicking Control Panel, clicking System and Maintenance, clicking Administrative Tools, and then double-clicking Event Viewer.‌ If you are prompted for an administrator password or confirmation, type the password or provide confirmation.

1. Expand Windows Logs, and then click System.

2. In the Actions pane, click Filter Current Log.

3. In the Filter Current Log dialog box, in the Logged list, select the time range.

4. In the Event sources list, select Power-Troubleshooter, and then click OK.

5. In Event Viewer, in the System pane, select the date and time for the event that you want to view.

6. On the General tab, view the Wake Source for the event.

After inspecting the filtered results, I see plenty of messages like so:

The system has resumed from sleep.

Sleep Time: ‎2010‎-‎02‎-‎24T23:42:44.283678200Z
Wake Time: ‎2010‎-‎02‎-‎24T23:43:10.233041900Z

Wake Source: Device -USB Root Hub

However, I've also spotted this rather annoying "unknown source" variant:

The system has resumed from sleep.

Sleep Time: ‎2010‎-‎02‎-‎20T22:41:30.687040200Z
Wake Time: ‎2010‎-‎02‎-‎21T10:07:54.488171000Z

Wake Source: Unknown

I will update my answer if the solution is disallowing this device to wake the computer.

Update:

I have resolved my issue!

Turns out that the "USB Root Hub" source is a little misleading; it was actually both my keyboard and mouse that were waking up my computer (without being touched/pressed). The solution was to uncheck "Allow this device to wake up the computer" on both the mouse and keyboard devices (selecting them independently does not resolve the issue).

Perhaps updating my mouse/keyboard drivers will fully resolve the issue.

I had this exact same problem on the exact same motherboard. But in my case it was just the mouse doing it; the keyboard couldn't wake it up if its life depended on it (and I have it set to do it, too). Probably because I'm using a PS/2 keyboard, not a USB one, and the auto-wake-up seems connected to USB hub weirdness. Or something.

I think I'm going to look for a BIOS upgrade now.

At another article, I saw two great commands to run in command prompt that will give more specific answers:

powercfg -lastwake

and

powercfg -devicequery wake_armed

My Windows PC was having the exact same problem. Turned out to be I had a USB phone charging cord hanging out of one of the PC USB ports with nothing attached. Removing the cord and only using it when I needed to solved the problem.

My answer was a different one: The Samsung Magician application.

I used the answers here to look in the log, and mine was "unknown" which obviously didn't help much. I disabled devices, etc., nothing worked.

And then I ran across "Windows 8 wakes up IMMEDIATELY from sleep" over on Windows 8.1 IT Pro forums. And so I did the same and stopped running Magician. And my problem was solved. Sleep works fine.

So if all else fails, and you have a Samsung SSD, disable the magician application.