why does mac pro reboot repeatedly after ventura 13.2.1 upgrade

What do you mean by "reboot repeatedly" ?

It is possible that a device may restart/load the Apple logo a few times during an update.

Hope this helps

First, are there any error/crash code when the device informs you about the crash ?

You should check if there are any running process (applications/tasks) that cause your machine to crash. Open Activity Monitor and see if there are any unusual stuff going on. You should also run a diagnostic to see if there's any hardware/software related issue Use Apple Diagnostics to test your Mac - Apple Support. Ultimately, if none of these help, I would reinstall macOS, but before that, I recommend everyone to reach out to Apple Support Team.

To look for possible software issues run the third party app EtreCheck which will provide us with important system details which may give us a clue. Post the report here using the "Additional Text" icon which looks like a piece of paper.

You may also want to try disconnecting all external devices in case one of them is causing a problem to see whether the issue still occurs overnight with no other connected devices.

We have many issues in our environment like this, after ventura upgrade device goes to reboot loop, only solution we could suggest is to clean re-install OS. it's not a feasible, can you please give any hint

Re-installed Ventura OS disconnected everything. Ran diagnostics. Ran disc Doctor repair. Reformatted hard drive yet my 2019 27 inch iMac still reboots every few seconds on start up. After spending all day rebooting it allowed me some time to run theses tests as I managed to start in safe mode very occasionally. I have given up on Ventura. Currently running happily on Big Sur. Will try Monterey (which it ran happily on previously) today.

We also upgraded to Ventura 13.2.1 about 2 weeks ago. And it restart repeatedly, inclusive after disk format and reinstalling the macOS.

Here -> iMac Retina 5K, 27-inch, 2020.