Jesus, how many times has that been answered.
The essence of it is that it was fake out to the audience. They presented her as a strong transformed character. They went over the top last year showing that she was smart now and crafty enough to come up with a convincing lie on the spot to save Littlefinger, and changing her look which was so in your face it was cheesy at the time. It was her decision to go to Winterfell to avenge her parents, implying she was ready to get involved in some dangerous shit to do it. Littlefinger told her something along the lines of "You've learned a lot from me, you can take this boy, make him yours." In the same episode, she told Miranda "You can't frighten me, this is my home."
It was set up to appear as though she was on an arc of vengeance, possibly play the Margaery role in the Margaery/Joffrey relationship. You never know, maybe LF was telling the truth and he was falling for her. Perhaps since she was a high born that he needs for the north, he would treat her differently. She was going to seduce him.
Nope, fooled you. Not only didn't she manipulate him, she didn't try. She froze up like a deer in headlights and started talking about how kind her previous husband was for not touching her. She did not even have the nerve to tell Theon to get out of the room. She's not going to manipulate him. She's a frightened rape victim and prisoner.
Of course it's the most realistic thing to happen. A teenage girl about to have sex for the first time and have it be with a psycho should be terrified. But this is the same episode that had a fight straight from Xena Warrior princess. Last year, little kids were shooting fireballs at skeletons. Meera last year was about to raped and was unrealistically saved at the last second by arrival of Jon. It's tough to know when they're going for hyper realism.
Now I'm not saying the swerve was bad by itself. That's actually a pretty good troll on the audience. Simply the fact that it's shocking does not make it bad. The questions are does it make a good story for her to be a helpless victim again? Was it done just for shock? Are the audiences getting tired of being tortured? Is too much to rape a character that the audience has watched grow up and be shit on for 5 seasons? Did it make sense for her to agree to marry into the Boltons?