I got a sudden fan shut down every time I would switch on my MacBook Pro (16-inch, 2019) after I upgraded macOS from Catalina to Big Sur. Here's how I fixed it.
CMD + R
- Press on reboot to enter Rescue ModeUtilities › Terminal
kmutil trigger-panic-medic --volume-root /Volumes/Macintosh\ HD
This is the Kext extension that, apparently, was causing the issue.
cd /System/Library/Extensions/AppleThunderboltNHI.kext