Genre Matters has some thoughtful, er, thoughts on why recuts work:
At their most basic level, these trailer remixes are all about genre; about leveraging the semantic elements of one genre into the syntactic elements of another. In most cases the humor comes from the interaction between the viewer’s knowledge of the original source film, or at least knowledge of that film’s generic underpinnings, and the reversal of expectations that comes with the re-edits. Usually, music (a semantic element in and of itself) plays the key role in shifting the focus.
The full post is here.