Where's it from? Shooting from the hip, in the literal sense, allows a gun-wielder to get off a shot quickly, since he doesn't have to raise the gun as far from his holster. With the gains in speed, though, come losses in accuracy. Through the mid-20th century it became a political metaphor for someone who speaks their mind confidently (and sometimes without tact.) It got a lot of play in political writing during the Reagan years given his decisive style. (His habit of starring in Westerns didn't hurt either.) "His occasional tendency to shoot from the hip on foreign affairs may be his most vulnerable point," wrote The New York Times of then-candidate Reagan in 1980. It then made a natural metaphor for George W. Bush, also prone to act decisively, speak too soon (in a Texas twang.)
Why it's catching on: Well some have commented on Perry's physical and philosophical position as the "perfect genetic splice" of Reagan and Bush, so it's no wonder journalists fall back on some reliable metaphors for the cowboy-like presidents. Perry's rural Texan roots, his cowboy-chic fashion (okay, not that chic), and, of course, his tendency to speak his mind all make the metaphor an obvious choice for a blogger on the run.