The marriage between Torvald and Nora has, in a way, always been dwindling on a rope. It is arguable that, perhaps, the biggest catalyst was Torvald's time of sickness, which corresponds to the time that Nora made the indiscreet deal with Krogstad; a deal that consisted in her borrowing money from him with the condition that Torvald never finds out about it.
It is safe to assume that, after Krogstad turns around and begins to use Nora's secret to blackmail her, Nora's attitude must have begun to change. This is evident in the way that Nora uses all kinds of escape mechanisms to avoid confronting the reality that her reputation, her marriage, and her lifestyle depends entirely on the promise of a man not to tell that she has committed what is thought of as a social faux pas. Among these escape mechanisms are the incessant shopping, the pretense that she is always happy, the excessive attention paid to Torvald (as if trying to make up for something), and the insistence that she keeps on playing the role of a doll.
Had Nora been a woman who feels fulfilled, she would not have the need of shallow victories to give herself any validation. However, things get worse for Nora when the moment of truth comes, and her husband finds out about the deal with Krogstand after all. Act III shows the cruelty with which Helmer treats Nora, without even giving her the benefit of the doubt:
HELMER: [reeling]. True? Is this true, that I read here? Horrible! No, no—it is impossible that it can be true.NORA: It is true. I have loved you above everything else in the world.HELMER: Oh, don't let us have any silly excuses.NORA: [taking a step towards him]. Torvald—!HELMER: Miserable creature—what have you done?Imagine the shock when Nora, who thought that Helmer would feel proud of her sacrifice, listens to the words "miserable creature" uttered out of her husband's mouth. She, after living in fear and anxiety for so many years after doing the deal with Krogstad, now realizes who is the person for whom she did so much: someone who is not capable to even consider the extent of her enormous deed. This is a sour moment, indeed. So sour, in fact, that Nora is willing to do one more sacrifice, this time for herself: to run away from everything, even on pain of social disrepute, to re-discover who she really is, what her life is really worth, and how she has come to this particular point in her life. The need for self-contemplation in Nora is a result of the deep dent that her husband's rejection has caused in her psyche. It is a disillusion strong enough to grant the huge undertaking of starting over,without fantasies and without lies. Therefore, it is the sacrifice made by Nora, met by the ungratefulness of her husband, what make the basic elements of the turn of their relationship, from a happy one, to a sour one.