Spinoff fad
From YTMND
A spinoff fad is one in which the first known instance of the fad was in a site that was part of another fad. Sometimes, spinoff fads die quickly; other times, they eclipse the original fad in popularity.
Examples
One of the most famous instances of a spinoff fad is Future Conan, which spun off of Conan has GONE INSANE!, which was part of the Conan is... fad.
Another notable instance of a fad coming from a nother fad is Lex Luthor, where the first site made was part of the PS3 fad.