Why does the win8 automatically reboot after the shutdown?

According to your description, your notebook is brand new and after the boot use everything is normal, should not be the cause of the hardware. Individual estimates should be system setup or the cause of the software.

On the software side, it is recommended that you restart press F8, select the last Known Good Configuration, or enter Safe mode to uninstall the most recently installed drivers and software.

System setup is most likely, win8 and Win7 compared to boot speed, in order to improve the speed of win8, Microsoft added a "hybrid boot" concept, that is, the operating system into a sleep-like state, while reducing the memory of the staging of data writing, Significantly shorten the boot time when the hard drive reads and initializes. This is the principle of the "Enable Quick Launch" feature.

Workaround:

First, in the control Panel you-> Power Options-> system settings, find "Shutdown settings", remove the "Enable Quick Launch (recommended)" Before the check, this will solve most of the win8 shutdown after the restart problem.

Second, restarting discontinuities may also be allowing hardware devices to wake up the computer. Open the Device Manager for your computer, right-click on the property, find power Management, disable this feature, and disable the keyboard, touchpad.