I think it's possible that someone (either Graham or Pierre) may have run the wrong route and been in the totally wrong place. They showed a shot of Brees on the bench immediately afterward, talking to Payton, and he was making hand gestures with his hands, like cuts and shapes of routes, trying to tell Payton what happened, and he was really angry about something. Graham was going deep with his back turned, Pierre runs to the middle of the field and claps his hands ("I'm open!") but then Brees throws it to a spot where NO ONE is, that is almost precisely between the two of them. You're right, it's bizarre..