ACTION : 'STARTUP' DATABASE USER: '/' PRIVILEGE : SYSDBA CLIENT USER
By default, Oracle will stop and start the instance when the server is shut down and started back up. Check your server's Event Viewer to see if the server was rebooted.
An admin can also start and stop Oracle by simply restarting the OracleServiceSID service in Control Panel->Administrative Tools->Services. Make sure you system administrators are not stopping and starting this service.