Why is my computer automatically starting up after I've shut it down?

Solution 1:

This issue is caused by a new feature of Windows 8 called Fast Startup which puts the computer in a special state called hibernation shutdown. It seems that some hardware configurations do not like this.

How to disable Fast Start:

1. Open start screen, and move your mouse to the right pane.
2. Enter "Power Options" in the Search field.
3. Click Settings and select Change what the power buttons do from the left pane that appears.
4. In System Settings, scroll down to the Shutdown settings section and uncheck Turn on fast startup(recommend).

Ref: Delivering fast boot times in Windows 8; What Is Fast Startup In Windows 8 & How To Disable It