The BES database backup is automatically done by the ppadmin user via cron.
As ppadmin user, run the following command:
This will show the users scheduled tasks like below:
00 22 * * * csh -c '$PPHOME/bin/backupDb'
The backupDB script is run at 10pm every day. By defaault it will backup to whatever your 'ppdump' backup device is set to within the database. By default this is the ppadmin environment variable $SYBROOT/backup directory.