Why is Peggotty angry with David's mother in David Copperfield?

David Copperfield gets the idea that Peggotty, his family’s live-in servant, is “cross” with him when he asks her about marriage (Chapter 2). The boy is trying to find out if a person can remarry after their spouse dies. When he asks her, she replies evasively and he interprets her manner as being “short” with him. He asks, “You an’t cross, I suppose, Peggotty, are you?” By way of reply she squeezes him tight. A few minutes later, his mother returns home with a gentleman David had seen at church. David does not care for his “ill-omened eyes.” After he leaves, David falls asleep in a chair and wakes up to hear his mother and Peggotty arguing. Peggotty seems to think his mother is already engaged, which she denies, accusing the older woman of being “bitter” and “unjust.” Peggotty expresses her disapproval of this man by saying that “Mr. Copperfield wouldn’t have liked” him. As things develop between them, the man—Mr. Murdstone—takes David to meet a friend of his. When David reports back to his mother the crass way the man spoke about her, she asks him not to tell Peggotty because “she might be angry.” Worried about finances and her and David’s future, Mrs. Copperfield ignores Peggotty’s advice and her own reservations and does marry Murdstone.
https://www.gutenberg.org/files/766/766-h/766-h.htm

In the story, Peggotty is angry with David's mother because she thinks that the latter is making some very poor choices.
In chapter 2, Peggotty tries to reason with Mrs. Copperfield. She tells Mrs. Copperfield that the man she is seeing will not make a fit husband for her. For her part, Mrs. Copperfield thinks that Peggotty merely wants to take control of her life.
On the contrary, Peggotty can clearly see from experience that Mr. Murdstone will be a cruel husband. Essentially, the faithful servant tries to warn her mistress, but she will have none of it.
Peggotty also begs Mrs. Copperfield to think of David and how things may change for him if she marries Mr. Murdstone. For her part, David's mother remains unconvinced that she needs to stay away from her admirer. She rationalizes her behavior and argues that Mr. Murdstone has yet to discuss marriage. However, Peggotty tries to warn Mrs. Copperfield that marriage is Mr. Murdstone's goal.
As for David, he takes an immediate dislike to Mr. Murdstone when he meets him. Later, we discover that Peggotty's instincts have been right all along. Mr. Murdstone does marry Mrs. Copperfield, and he does make the lives of his new little family miserable.