Propagation by social suggestion.
Do you remember ever seeing a picture with a girl posing in duckface before the internet and social networking sites existed? I don't. Girls are conditioned by the mere sight of their peers doing it, whether consciously or not.
The origination point of the duckface probably stems from the idea that in order to justify taking a picture of oneself without looking like a self-obsessed egotist, one has to add something to the picture that is either "funny" or "cute" so as to appear as the motivation for taking a picture in the first place, rather than showing off physical attributes (which is almost always the real reason for taking an "edgy" angled shot in the mirror). This behavior then extended to other situations such as group photographs by nature of being a practiced action.