I used to run into a problem in which my newly built computer would power on itself after being shutdown. Most of the online posts talked about Windows Scheduled Tasks. However, these come into play only when the computer is in 'Sleep' mode not in shutdown.
Finally, I found the solution. The problem was a default setting on Asus motherboard (Z87 Pro) which kept the Wireless adaptor powered on even though the computer was shutdown. This would cause the box to boot up anywhere between 2 and 20 mins after shutdown.
The solution was to enable "ErP Ready" under the APM Configuration in the BIOS. When the machine shuts down with the Wifi adapter enabled, the Wifi LED will go off (instead of staying on like before). The ErP mode setting will disable the Wifi wakeup at the BIOS level. The setting description says, "This item allows user to switch off some power at S5 to get the system ready for ErP requirement."
S5 state is the shutdown/OFF state. So it makes sense when it says it switches off power at the S5 state; it switches off power to the Wifi module, essentially disabling it and not allowing it to turn the machine on.