Wanted to share this in case others have this problem.
After upgrading to BackupExec 21.2 we started getting unexpected reboots during some backups. It wasn't a problem previously and even upgrading to BackupExec 21.3 didn't resolve this problem though it may have helped.
Seems the issue has been with the agent on a small number of servers and uninstalling the agent and re-installing resolved the problem so far. After uninstalling I did remove the Veritas files under Program Files and ProgramData. Not sure it was needed but it's what I did.
So far I've only seen problems with Windows agents. Two of the servers were Exchange 2019 servers, one was a hyperV server, and another was just a web server (all Windows 2019).
Have not seen such a case in support with those version but I know that sometimes the AV may cause such a problem. To start with, just ensure that the exclusions (https://www.veritas.com/support/en_US/article.100046324) are set correctly but as you said that after reinstall the problem has not surfaced so not sure. Does event viewer give any clue about it. can check the memory dump in case it got formed when the reboot took place.