I know this is a pretty old post, but this is something i've been thinking about ever since I first watched the movie over a year ago... Several people who worked with Jon have noted that he was great at holding grudges, but I definitely don't get the impression he had a very sour relationship with anyone on set besides Caroll. My theory is simply that Jon was jealous of Caroll. He didn't like the fact that Caroll gave Oscar a heart of gold. As Jon said in an interview that was seen on the movie, "He fights very hard against my concept of Oscar." Oscar grew a lot as a character after the first season, as did Big Bird, both largely because of suggestions made by Caroll. Jon was one of the main creators of them, so the fact that Caroll took them and helped make them into something better might've angered him. But that's just my take on things.