Going Along With The Crowd

Those with Narcissism, and also those who simply lack maturity, tend to judge people and things by how other people act, and what other people say.
So if they liked a new person at first, but later it looks like they person is not popular, they will usually reject the person.

It's about being accepted, liked, admired, or feared; most N's want to be associated with "popular" or powerful people, and so will reject and avoid "less popular" individuals in order to avoid being associated with them by the crowd.

One of the main reasons Narcissists will make up cruel rumors about a Target is to avoid being seen as a "popularity whore". They have to make up an excuse to COVER themselves for rejecting the person, so they try and spread it around that the person has 'bad character'.
(Of course, smear campaigns only work when there are other willing participants who eat the slander up like ice cream, who are too immature to stand up for the targeted person and have their back.)