As early as Act I of A Doll's House we learn about the animosity that exists between Krogstad and Helmer, and also between Krogstad and Nora.
As far as Nora and Krogstad goes, the problem lays in that the two had made a deal where the latter would have lent Nora money that she needed during a time when Helmer was ill and needed treatment. The problem is that this type of business is a very bad idea from a social standpoint. Nora comes from a time in society when women were not to take steps into the financial status of the household. Even more of a faux pas is to enter in negotiations with any man. To make matters worse this man, Krogstad, is a subordinate of her own husband. Boldest still is the fact that she also expects that the man would keep their secret.
Aside from all this, Krogstad has a history with Helmer. They knew one another from a long time ago and were even quite intimate according to Helmer himself. However, Krogstad is not a likable character and Helmer is too sensitive. The two fell out and the friendship ended. Krogstad has been working for Helmer, but continuously disrespects the boundaries that employees and employers should uphold. This, and his lack of skills in other areas, have rendered him ready to get removed from his position at the bank. This is when things become problematic for Nora.
Krogstad [changing his tone]. Mrs Helmer, you will be so good as to use your influence on my behalf.
Nora. What? What do you mean?
Krogstad. You will be so kind as to see that I am allowed to keep my subordinate position in the Bank.
As expected, Krogstad will use the information that he has on Nora to apply pressure and make her demand her husband to keep him at his post. A scandal of that nature surely would signify the end of Nora's marriage, and the end of her as a woman, as society offered nothing else to her but the opportunity to be a wife and mother. This is why Nora is so scared when she hears her husband express himself negatively against Krogstad.
Nora [...] was it really something very bad that this Krogstad was guilty of?
Helmer- He forged someone's name. Have you any idea what that means?
Nora- Isn't it possible that he was driven to do it by necessity?
Helmer. Yes; or, as in so many cases, by imprudence. I am not so heartless as to condemn a man altogether because of a single false step of that kind.
Nora. No, you wouldn't, would you, Torvald?
The more angry Helmer gets, the quicker he would want to dismiss Krogstad from his post. This means that Nora's secret will come into the open and all chaos will break lose. This is why his attitude toward Krogstad scares Nora so much.