Short answer: we're not sure yet. Speculative answer: the MPAA Ratings Board probably had second thoughts about some of the content in the trailer, which they previously issued an "Approved for all audiences" green band. For example, late in the trailer, Zach Galifianakis' character uses a water bottle to simulate fellatio between an elderly Thai gentleman and a monkey. That such content was included before a PG-13 film may have caused some blowback for the MPAA, leading the organization to vigorously cover its own ass.
Neither Warner Bros nor the Motion Picture Association of America have officially commented on the trailer snafu, but the notice issued to theaters assures that a new The Hangover Part II trailer will be issued for placement on Scream 4, the R-rated sequel due on April 15th. The sequel to the highest grossing comedy of all time will showcase water bottle-monkey eroticism in theaters on May 26th. As you surely know, it stars Bradley Cooper, Ed Helms, Zach Galifianakis, and odd man out Justin Bartha.
UPDATE: The Hollywood Reporter has a statement from Warner Bros explaining what happened. It seems that, in its haste to get the trailer out there, the studio failed to fully clear their preview with the MPAA, and subsequently incurred the wrath of the ratings board for creating a green band trailer with words like "weenus".
To refresh your memory and test your delicate sensibilities, here's the trailer that's too hot for theaters once again: