What is the Roman Catholic Church doctrinal justification of its perceived opposition or support of the independence of Catalonia? According to the New York Daily News,   


  Vida Nueva (New Life) internet publication said the pope told Spain's new ambassador to the Vatican, Gerardo Bugallo, that the Holy See is against all self-determination moves that are outside decolonization processes.  


(I have to say that although this is circulating in the news I don't know if there is an official statement from the Vatican).

In Spain though, some 400 priests openly support Catalonian independence.  

I'm confused as to what business the Catholic Church would have in supporting or opposing an independence movement, unless it is related to its doctrine or the Bible.
the Holy See is against all self-determination moves that are outside decolonization processes.


The Catholic reason for this is fairly simple. The people don't have a right to overthrow or revolt against legitimate rulers. The reason that decolonization processes are good is that they take away a nation's unjust claim to another sovereign peoples. IE, the colonized land was gained via unjust military action. Those are illegitimate rulers.

The syllabus of errors, a papal encyclical listing statements to be condemned, contains this statement:


  
  It is lawful to refuse obedience to legitimate princes, and even to rebel against them.
  


That is, the above statement is condemned by the Catholic Church as false.