So I think I figured out the problem at least in my case. For whatever reason after I upgraded to big sur I found my fans were not kicking in when they were supposed to and as a result my computer temperatures ran really hot. I have turbo pro free extension that enables me to monitor my temp. So it would heat up, freeze and reboot. I would have nothing more than a Firefox browser open and temp would run upto 90 degrees. So to fix this I reset the PRAM and SMC. Once I did that all is well. Fans kick in and my computer temp is between 55 and 70 Celsius now. if this doesn’t work you can run diagnostic by rebooting your computer and pressing and holding down “D” on reboot. That will take you to run online diagnostics if the problem is something else. Hope this helps