How can any of us really know what playoff pressure is like? I remember back in the dramatic Game 5 of the 1986 ALCS when we came back against the Angels, a Red Sox relief pitcher named Steve Crawford who was brought in with the game on the line later said, in effect, that he was afraid he was going to lose control of his bodily functions right on the mound.
Also, how do we explain Jon Lester's inability to throw the ball to first base? And he's not the first player to develop the 'yips' in some form, either.
As I said about Price, I don't think it's a case of being a choker. But there are a lot of psychological things going on inside people, including guys who have been in the big leagues a long time.