As true a personality type as a person might have, they still have their own variables as far as individual personality, traits, and temper are concerned. It's been my experience that a person with NPD can actually have a very precise way of dealing with other people that doesn't involve losing their temper at all, though I'm not suggesting that's true for every person fitting the NPD diagnostic construct (or its subtypes).

Inasmuch as they prefer the feeling of being in control (as anyone does, but they are perhaps more tethered to their desire to feel and be in control), feeling not in control can cause them fairly intense emotional stress, whether or not that stress expresses itself specifically as anger.

Best way to find out what causes a person to lose their own temper might be to ask them. Just my take.