Clearly, the prepositional phrase with "behind" is functioning as an adverb in the sentence, much like "behind the barn." But I don't think that's what Marc's really interested in.
Most of the times I've heard it, "behind" has its original sense denoting a place relationship, or extended to Ken's sense of responsibility ("so that's who's behind all those robberies"). In this case, however, it seems to have a general sense of "related to," similar to the way I would use "over."
I'm guessing it's an innovation in the Baltimore dialect of Black English. From a corpus perspective, it turns out that the script to this episode of "The Wire" and many others are available online and searchable. You can find a bunch of other examples. It's hard to find with a keyword search unless you're looking at something that's specific to that dialect, because there are so many other uses of "behind."
From the same episode: "I'm gonna need a tetanus to get behind this bullshit." "Everything else in this country gets sold... without people shooting each other behind it." http://movie.subtitlr.com/subtitle/show/268875
From other episodes, "You just took a real detective off a real case behind your bullshit." http://movie.subtitlr.com/subtitle/show/138236#line531
"You're going to jail behind this shit. - Yes, you are." http://movie.subtitlr.com/subtitle/show/237503#line31
"It's my recollection that a city police got shot behind Mr. Barksdale's business." http://movie.subtitlr.com/subtitle/show/205769#line431
"You still fucked up behind that stickup, man?" http://movie.subtitlr.com/subtitle/show/172760#line51 "I nearly got killed behind this caper, you know?" http://movie.subtitlr.com/subtitle/show/172760#line251 "And I will put a bullet in all you all behind, what happen right now, you heard?" http://movie.subtitlr.com/subtitle/show/172760#line351
Outside of scripts for "The Wire," I was able to find one instance on an Oprah message board:
"was I better of useing herion than drinking at that time behind what happen with the other family" http://www.oprah.com/community/message/976102
-Angus B. Grieve-Smith
grvsmth at panix.com