I'm sure we've all been attacked by Dark Brotherhood assassins who hold a note from Astrid, saying the Black Sacrament has been performed and the client has paid and that failure is not an option. I always assumed it was a random occurence. Well! Today I picked Maven Black-Briar's pocket and found an interesting key called Maven's Personal Key, and immediately went to Black-Briar Manor to find the door it belonged to, which turned out to be one in the basement. Inside, it was obvious the Black Sacrament had been performed, and a note to the Dark Brotherhood lay in the circle: This made me wonder: maybe those assassins that come after you are on a contract belonging to Maven Black-Briar! I know it's probably all a coincedence, but I think it ties together well enough to be a plausible explanation.