backup syncing files

Hi I have an iPhone6 and my wife an iPhone6 . After backing up my wife's phone when I click the eject button, it takes a long time with a popup box saying syncing files and ultimately sync failed. This doesn' happen when I backup my iPhone6. Both are 64 GB and both are on iOS8.2

Why this difference in behavior?

After syncing files, the backup seems corrupted if I try to restore on a completely reset iPhone.