The short story is that Castiel was working with Metatron on a spell that supposedly would lock up all the एंन्जल्स in heaven, which Cas believed would give the एंन्जल्स time to sort out their issues. He succeeds in completely the spell, but Metatron reveals that all the ingredients he gathered was for a spell that would actually cast the एंन्जल्स out of heaven. Which is what happened at the end of season 8.
Because Medatron (How do आप spell that name? :P ) had a plan to cast all एंन्जल्स down to earth and for that he needed Castiel's grace if I remember right
