The problem with that call is that he had his back to the play so one could claim he wasn't actually all that involved with it. Yes, his arm was outstretched, but you can't say he was trying to touch the ball with his hand or arm because he was facing the other way. On the other hand (sorry!) none of that matters in a sense. His arm was away from his body and not in a "natural" position, and he was in the box. So clearly a handball for a penalty. The reason is took so long was that it's potentially debatable as to whether the ball just happened to hit him without him knowing anything about it. I think the deciding factor was that even though he had turned his back on the ball he was still actively defending, and in addition his arm was outstretched. So yes, a lot of possible room for interpretation, and a good few looks for the ref to actually decide to reverse something that had happened quite a while ago and that he allowed to play on.