What's at the other end of the cable? (is the USB port built in to your car, head unit, or is it a 12V charger).
the phone recognizes the charger as a computer due to signals on the data lines. The middle two pins/wires in the rectangular USB connector. You may want to look for debris in there as an intermittent contact may cause this kind of behavior.
Same here with iPhone 6. I think this is a bug in either iOS8 or iTunes.
I am connecting using the new cable that came with my phone directly to the USB port on my MacBook. Swapping cables does not help.
I don't get it every time though. I get it when I notice wifi sync stops working when iTunes can't find the phone over wifi. Then I restart the phone to get the wifi sync working again (this works sometimes) and when it does not I attach with the cable and get that message. The message would explain why the wifi sync stopped working, but no idea why iTunes is losing track of the fact I have authorized this phone to sync.
The odd thing I notice is when this happens, the Phone "name" goes back from my personal name to the default "iPhone" and I have to rename and sync to get the name back.