Talk:Episodes, films, and shorts/@comment-7454040-20131024153120/@comment-26973802-20131025180737

Because a clip was shown of Luna saying "Did you expect me to sit idly by while they all basked in your precious light? There can only be one princess in all of Equestria! And that princess will be me!" And transforms into Nightmare Moon.

Some take this as Luna being jealous of Twilight thus creating a second Nightmare Moon persona; others like myself take it as a flashback Twilight enters, probably while learning about the Elements of Harmony's history