Shot down is a bit strong -- there's no reporting that the helicopter was unable to keep flying. But they had a wounded crewman and no need to press on for a mission, so they landed ASAP.
As to who did it, at 1000 feet altitude plus slant range, hitting a helicopter deliberately with a small arm seems incredibly unlikely. I'm guessing it's actually an accident or at least someone who didn't actually mean to hit the helicopter but got really unlucky.