I RP that it is Maven who hunts me down for gaining so much influence in Riften to where (I RP this also) the citizens are not afraid of her as much anymore and she can't do anything to me outright because she knows she'd get torn the eff up.
Actually Maven Black Briar will hire the DB if the player chooses to do a certain quest in a certain way. It's one of the very few RPG moments in Skyrim with some tangible C&C. Her name will be written on the contract, though. These are random contracts probably paid by the families of the countless bandits and necromancer slain in dungeons by the Dovakhin.
I think the DB writes down the name of the employer on the contract for "insurance" reasons. In case the assassins fail, the blame goes directly on who ordered the murder rather than on the DB itself.