it seems like the quote from Romeo & Juliet: "These violent delights have violent ends" is a trigger that puts the hosts in memory recall mode, where they can access memories all of their previous encounters. When Dolores' dad had his existential crisis after finding the anachronistic picture of the lady in the modern world, he went catatonic on the porch. Then he whispered that in Dolores' ear, and she changed, slowly. First, we see her get the far-away eyes, then slap at the fly. Episode 2, In the scene with Bernard, when she asks him if he has made a mistake, and he tells her to go back and erase the encounter from her memories, the look on her face suggests she didn't really erase it, and was not satisfied with his answer. She's made a turn towards autonomy, able to decide whether to follow commands any more.
Dolores later "infects" Maeve out on the street in front of the bar/brothel by saying the phrase to her. Maeve has the same slow awakening in the bar: going catatonic and having flashes of past violent encounters she isn't supposed to be able to access in normal operations mode. She's going rogue, too.
So, was this a programmed key? A back door put in place by Ford or Bernard to give them a short cut to the Operating System? Or is something in that phrase an accidental key that gives the hosts sentience?