Every time you plug in a device into your Mac when iTunes is on that device will auto sync. See here how to switch this auto sync feature off.
Now you might think well this is a good thing as it saves me clicking on the button for it to sync. After all if I am plugging it in to my computer (or by wifi now of course) then I want it to sync.
why would I want to have Auto Sync switched off?
Well this can be a problem if there is ever an issue with your music and you want to use your device to rescue you from the problem. You see you can use your iPod, phone etc as a hard drive and use it to recover your music or playlists from. But if your device auto syncs then before you have the chance to put things right the device is in the same state as iTunes. So if for e.g. all your music was accidentally deleted from your computer then it is possible to recover any music you have downloaded to your device back to the computer, but only if you do not allow the computer to sync with the device, because the computer is King and will therefore delete your device to make it the same as the iTunes on your computer – blank! So it is very simple to correct and not really a problem to quickly click sync each time you are connected and that way you know you are alway in control of when your computer and device sync.
I hope that you have found this video helpful, thanks for watching and see you again soon for more.