A- does not mean something is wrong, it's just not top notch perfect to his standards. I had some professors who never gave 100% because they thought only God and them knew the subject so well.

Asking to explain why A- may come of confrontional. Calling somebody who has finished theire studies already and works in academia really is unprofessional and disrespectful. You can say things like that to your friends, but never on academic ground.