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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.

B
 
I would say it is how your payer in your car is handling it, anotherwords it sees your HU in your car more as a computer more then anything.

my Pioneer in my car work just the same with my IP6 (IOS 8) but it is just a basic cd player with usb port. You may have a more complex system then me.
 
This has been happening to me when I plug into any computer since the last iOS update (or maybe even before).
 
Every time I charge my phone in the car it now asks "trust this computer".

It asks me every time I charge!

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.
 
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