I want to know the answer to this question, too. I'm pretty sure my mother has NPD, or is at least on the far end of the spectrum.

I remember times when I would make a personal decision she didn't like. If I felt really strongly about it, I wouldn't care what she thought. It enraged her. She would go on a passive-aggressive rampage about it. It didn't matter that it was a choice that was mine and mine alone. She could not handle me having my own mind. Someone who is confident is less easy to control. So maybe people with NPD do feel threatened by confident people?