My gut feeling is that you had a hairline crack in the tranny case to begin with, OR the adapter was not bolted tightly enough (or came loose) from the back of the tranny, giving the tranny a chance to start spinning free from the adapter????? Too many guys running your same set up that have never cracked a tranny like that. Look for black grease inside part of the cracked metal, where grease was laying inside a hairline crack before the dog gone thing gave way. That exactly happened to me, but all the way around the bellhousing- - - black grease inside the first 3" of a 9" crack.