Mac mini restarting on its own.

Do you have any 3rd party apps that claim to optimize, fix, clean, or protect your Mac? Any Anti-Virus? (All of which are known to cause system problems in macOS) If so, you should remove those per their individual removal instructions. If you don't, try booting into Safemode then back to your default account and test to see if the problem goes away.

Start up your Mac in safe mode - Apple Support

My solution: My late 2012 mini running Catalina appeared to be overheating and repeatedly rebooted. I simply Installed and ran a fan control app (Macs Fan Control) at full blast. Also elevated the mini about 1/2 “ on its center plastic stand… this enhanced the air flow. Seems to fix it.