I have this same issue on my AVH-4200NEX, I'm not sure if it's my phone confusing the unit, I turned off the auto turn on Pandora function via the Pandora app. I'll check later when I get more time to play with the radio and report back.
Edit: Not the problem...anyone else?
Edit 2: Apparently this is an issue Pioneer is aware of and they are working with Pandora for a solution, no eta on the app update to fix though.