GA-Z87-D3HP running Linux rebooting after powerdown

[sacentre]

Can anyone suggest a fix for this?

My GA-Z87-D3HP mobo running Linux reboots automatically about 15 to 20 secs after a powerdown from OS.

I've tried many different versions of Linux with the same result so I'm pretty sure it's not the OS. Also, I've looked at settings in the BIOS but cannot see any config setting that controls this behaviour. 

Thanks in advance for any advice.

System Info
Linux Mint 18.1 Cinnamon 64-bit (and others)
Intel Core i7-4770K @3.5GHz
16GB RAM
On board graphics

[shadowsports]

I seem to recall reading about similar behavior from another user over a year ago...  Can't find the post.  Do you have WoL enabled?  Are you using system power on/off by schedule in BIOS?