It it’s in sleep or hibernate mode then there is a possibility that something is triggering it to start up. Like the mouse getting bumped or some other electrical issue. The may be some background virus causing it to wake as well. I hope it’s not booting up from a power off situation. If that is the case, take it to a repair shop.
Another possibility is that this may be caused by a Windows update requiring a reboot. Check you update settings to see when they happen and get applied. These normally occur during down time or when you get ready to shutdown. You may see a message that updates are ready to be applied. You will have options to apply and shut down or apply and restart most of the time.