I know their are speculation that Maven is the one because of random encounters with the dark brotherhood assassins that don't occur till after you leave riften for the first time. But here's another speculation that Hadvar is responsible instead.
During the civil war quest he gives you the i heard you had died speech and he insults you for being the dragonborn. So this could be his way of trying to tie loose ends involving you because of you originally sent to be excuted in helga. Someone like him would not tolorate a dragonborn in skyrim because of the power a dragonborn can possess and that poses a threat to hadvar
And remember in the note from your unknown ally he/she mentions not everyone is anxious for the dragonborns return.