Why did the 天使 fall in the finale of the season 8?

The short story is that Castiel was working with Metatron on a spell that supposedly would lock up all the 天使 in heaven, which Cas believed would give the 天使 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 天使 out of heaven. Which is what happened at the end of season 8.

Because Medatron (How do あなた spell that name? :P ) had a plan to cast all 天使 down to earth and for that he needed Castiel's grace if I remember right