They got hurt
Narcissists have a completely different attitude as compared to others. If they get hurt by any means or believe that they have been mistreated, they might most likely block you. Narcissists believe that if someone hurts them, they cut connections with them to feel good.
It’s a form of Devaluation
If narcissists get ill-treated by someone, they might want to devalue them. They have this nature that by devaluing someone, they feel good about themselves. Narcissists think that the person who hurts them is worthless and should be undervalued. For them blocking someone is similar to disparaging.
To make you feel worthless
To feel good, narcissists try to hurt people’s ego and discourage them as much as they can. They do this to people who hurt or harm them. In situations like this, they block people to make them feel useless and unimportant.
They can even block people they think can mistreat them also if they have done nothing yet. If they postulate that this person can ill-treat them any time in the future, narcissists will make sure that they block them. Their assumption or wrong perception leads to this action.
You ignored them
Ignoring narcissists hurt them more than anything, and if they feel someone has ignored them, they would straightaway block them. In fact, if they perceive that this person might ignore them, then also they can block them. If unintentionally, you did not wish them on any special occasion, they would block you. (See What happens when you Reject a Narcissist?)
They decided to cut connections
If they block anyone, that means they have decided to amputate all connection with that individual. This does not mean that they would cut off the links permanently. They might unblock you or restore the relationships if they get the treatment they long for.
To send you an indirect message
This is again something that you can expect from a narcissist only. If they block someone and want that person to know about it, in this case, they send an indirect message stating that they do not mean anything to them.
To Manipulate you
The narcissist does not like to work as per others’ rules or dominance. In this situation, they might block the other person to put pressure to work as per them. (See What Makes a Person a Narcissist?)