You may not have turned off Wake on Network. There is a Trojan horse program in your computer, which will be opened through the network card. Please test it like this. Unplug the network cable after the computer is turned off, and if it will turn on automatically, that's why. On the one hand, you can kill Trojan virus, on the other hand, you can turn off network wake-up support in BIOS settings. Specific BIOS settings can be found in the motherboard manual.