Pope Francis lays down the law when he sees fit.  The whining over this is really a bunch of priests who thought that they deserved to be bishops and were mad that they weren't appointed bishops.  It is clericalism on acid.  There is nothing to suggest that the bishop in question has any scandals associated with him or that he is unfit to be a bishop.  This is basically a selfish tantrum on the priests of this diocese who wanted the promotion for themselves.  (And yes, Pope Francis does favor priests to become bishops from the dioceses they are incardinated in, especially those who are pastors.  However, I doubt that he thinks any of these Nigerian priests are fit for promotion after their tantrum.)