Might be
Bug 20785 - Full backup starts everyday
(Not 100% sure as the key to identify that was 0 account fulls and you're 1 account fulls.)
Probably time to upgrade from 5.0.1 to 5.0.5
Of course after a good zmbackup -f -s server.domain.com -a all
A copy of your /opt/zimbra and any linked directories would 'play it safe'.
5.0.5 is Released!