Why do I get the message "iTunes could not back up the iPhone "x" because the iPhone disconnected."

I frequently get this message when I plug in my iPhone 6 to my MacBook Pro, i can sometimes get it to work by disconnecting the phone from the USB cable, closing all the apps on the phone, restarting iTunes on the Mac, and the plugging the phone cable back in. Manually hitting the Back Up Now button in iTunes will usually result in a back up. But this should be a simple thing ... plug in the phone via cable, iTunes backs up the phone and completes a sync. Why not?

All my software is up to date. Details:

• MacBook Pro (Retina mid 2012}
• OS X 10.11.3
• iTunes version 12.3.2.35
• iOS 9.2.1