To Bel-Air someone is to tell them a shaggy dog story, supposedly relating to a serious matter, and then end by inserting the lyrics from the TV show "Fresh Prince of Bel Air" 
(from encyclopediadramatica.com)
and this is the reverse Bel-Air, where you START with the song and then segue into something serious. 
(from http://xkcd.com/)
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