In my humble opinion, I don't see why it's anyone elses business WHY you might have left your other pdoc. If the question comes up, tell the new guy, "There were some ethical concerns that I couldn't abide." Simple, concise, and also lets him know that you expect things to be on the up and up. You aren't looking for a confidant, a friend, or a therapist. You want him to manage your medication. All he needs to know is your medical history, what meds you've taken, any dx'es that are pertinent, etc.