Getting “SQL Server Alert System: 'Alert Severity 20' occurred” error occasionally
Check the SQL Server error log and you will find a corresponding log message that includes the IP address of the originating system.  This will be the most important clue to investigating what is causing this.

It is most likely that this is being caused by security software that is scanning/probing your server for vulnerabilities.  If you find that is not the case, then it is either something that is accidentally or maliciously attempting to make a connection to whatever port SQL Server is listening on.