This could be due to a few different reasons but this is probably due to configurations with the ***''fast boot''*** option in the bios, uncheck this option to ensure the pc can shutdown without restarting its ***''previous state''***.
Double check your alert wake up timers and/or check the wake on LAN settings as well but check your bios settings for the option I mentioned previously that will solve the autorestart, also you could try holding the power button for a force shut-off but check the fast boot option first and then if you have issues I will assist.