I thought you said something similar in your PU guide though :sad:
I was playing as Scandinavia in the 1600s, with a RM with France (Valois), Austria (Habsburg) and Lithuania (Jagiellon). I was alone in my dynasty, Paus (Norwegian). My rivals were England, Portugal and Sweden.
http://cloud-4.steampowered.com/ugc/545261868589260466/ADA221D9C640603331721F7E8F41DAA7F9657E4C/
Suddenly in the 1630s, Castile got PU'd by Portugal, I received a popup and chose to contest the succession. With 60% warscore I managed to steal the PU from Portugal.
http://cloud-4.steampowered.com/ugc/545261868589188334/AAB3989000A2CFC9CB5BE77DF1DF9E156038A727/
http://cloud-4.steampowered.com/ugc/545261868589230018/988AF8D5ADCC459DA9BCBE38A2C9D242DE05EC7C/
THis was because Portugal had you as a rival, not because you had Portugal as a rival.