I think it's connected with St. Peter; he as an early martyr was crucified upside-down since he thought himself unworthy of dying the same way as the jewish carpenter before him. The St. Peter's church/cathedral in the Vatican is supposedly built on top of his grave, and more importantly; in catholoic thought the pope´s authority is "inherited" from Peter as he was made "deputy" by Christ which was thus inherited by the early bishop in Rome in his stead. A "peter's cross" in Swedish denotes an upside-down cross, so perhaps it's worn by the pope in honour of St. Peter, but it looks prett strange yes