I think it is a combination of things. Shaky arm, poor plate blocking, and his ability to frame a pitch is very much lacking. Pitchers don't have much confidence in him. It would be one thing if it was a small sample size, but throughout his career, pitchers have done significantly worse when pitching to him than opposed to the other catcher on the roster. Relief pitchers have a very different mind set-- there is no settling in, or figuring out which pitches work, they just throw the ball, and hope their stuff is good for that given night. That, and Bobby Valentine is a matchup genius
Not to mention the abysmal OBP and strikeout rates.