Amanda:Why does Amanda sometimes promote full dumps?
From The Open Source Backup Wiki (Amanda, MySQL Backup, BackupPC)
Amanda promotes full dumps (performs them ahead of schedule) to spread the full dumps along the dumpcycle, so that all daily runs take roughly the same amount of tape and time. Without this technique, full dumps would all occur at the same part of the dump cycle.
As soon as you start using Amanda, it will run full dumps of all filesystems. Then, on the following runs, it will promote some backups, so as to adjust the balance. After one or two dumpcycles, it should stop promoting dumps. You can see how well it is doing with amadmin(8) <conf> balance. If you find the results surprising, you may want to adjust dumpcycle or runspercycle, and/or dumptype parameter maxpromoteday.