PC turns back on after shutting off

[HuskyJ]

Lately I have experienced a rare occurrence. For some reason after I shut my pc down it will turn back on immediately. This only happens in rare occasions. So far I have experienced this twice, once last week and another today. It turns off alright in both the user login screen and while working with any user profile. I still have no idea why this happens as this didn't happen with my laptop or any other pc I've used, and since it's only happened twice so far I can't recall what both events had in common.
Maybe it isn't anything but the OS acting up but I just want to make sure it isn't a big problem.

Specs:
OS:Windows 7 64bit
Processor: Intel i5 6600
MoBo: Gigabate B150 HD3
GPU:Gigabyte GTX 750ti 4GB
PSU: EVGA 600w 80plus
Ram: Kingston Fury 8gb
Case: Thermaltake Versa H21 mid tower

 

[Outlander_04]

It may be waking/ starting because of your network .

Check in BIOS to see that LAN , WLAN and pci-e events can not start the computer .

 

[HuskyJ]

I checked the BIOS and the network events are turned off. The wake computer on shortage/sudden shut off is turned off too. Today I found that the common factor on these occurrences is that it only happens if the computer was put on sleep mode during the session. It doesn't happens with hibernation, reset, or any other option except sleep.