There are three primary reasons that narcissistic rage happens.
Injury to self-esteem or self-worth
Despite an oversized opinion of themselves, people with NPD are often hiding self-esteem that’s easily injured.
When they’re “hurt,” narcissists tend to lash out as their first line of defense. They may feel that cutting someone out or intentionally hurting them with words or violence can help them protect their persona.
A challenge to their confidence
People with NPD tend to try building up confidence in themselves by continually getting away with lies or false personas.
When someone pushes them and exposes a weakness, people with NPD may feel inadequate. That unwelcomed emotion can cause them to lash out as protection.
Sense of self is questioned
If people reveal that someone with NPD isn’t as capable or talented as they may pretend to be, this challenge to their sense of self may result in a cutting and aggressive outburst.