ÂÂ Once iPhone has been configured to sync with a particular computer, iTunes
automatically makes a backup of iPhone on that computer when you sync. iTunes
wont automatically back up an iPhone that is not configured to sync with that
computer. If you have configured iPhone to automatically sync with iTunes on a
specific computer, iTunes will back up iPhone every time you connect it to that
computer. Automatic syncing is turned on by default. iTunes only makes one backup
each time you connect, even if you sync multiple times before disconnecting.
Looks to me like the way around it is to do the manual syncing, which is what I have always done anyhow. I hope this helps you.