Why does Mrs. Hale remove Minnie Foster Wright's erratic stitches?

In Susan Glaspell's Trifles, Minnie Foster Wright has been arrested under suspicion of murdering her husband. A host of characters, including Mrs. Hale, arrive at the Wright residence in order to search for evidence of the crime. While the men walk from one room to the next, Mrs. Hale and Mrs. Peters stay downstairs and discuss the state of affairs. When they find a quilt that Mrs. Wright has been making, they notice that, unlike the rest of it, the most recent section has been stitched together in a chaotic manner:

MRS. HALE: [Examining another block.] Mrs. Peters, look at this one. Here, this is the one she was working on, and look at the sewing! All the rest of it has been so nice and even. And look at this! It's all over the place! Why, it looks as if she didn't know what she was about!

[After she had said this they look at each other, then start to glance back at the door. After an instant Mrs. Hale has pulled at a knot and ripped the sewing.]

MRS. PETERS: Oh, what are you doing, Mrs. Hale?

MRS. HALE: [Mildly.] Just pulling out a stitch or two that's not sewed very good.

The look that each woman gives the other is a clear indication that they believe they have found evidence against Mrs. Wright. When Mrs. Hale begins pulling out the stitching, Mrs. Peters states that they should not be touching anything.

While the quilt represents evidence against Mrs. Wright for the murder of her husband, there is another layer to Mrs. Hale's decision to fix it. Previously in the play, the male characters spoke of and mocked the state of Mrs. Wright's house:

COUNTY ATTORNEY: Here's a nice mess. . .
Dirty towels! [Kicks his foot against the pans under the sink.] Not much of a housekeeper, would you say, ladies?

After the men comment on the dirtiness of the house and about women in a condescending manner, Mrs. Hale and Mrs. Peters discuss the possible reasons for the mess. It is clear that the women find it unfair to judge a home when the owner is not there to defend it. In addition to hiding the evidence, it is apparent that Mrs. Hale fixes the quilt to both help Mrs. Wright and avoid giving the men yet another thing with which to find fault.

As to the part of your question about why the quilt was stitched wildly in the first place, the audience is left to interpret that reason themselves. By itself, a quilting mishap does not seem like such a strange occurrence. However, as the play progresses and more evidence is found, it becomes clear that Mrs. Wright was in a very nervous and agitated state while working on the most recent section. There are two possible explanations for this: first, she was at a point where she was fed up with the treatment from her husband and her life in general; and second, she may have stitched it after murdering Mr. Wright. Either way, her mind was preoccupied, and this lack of appropriate attention led to the erratic stitching.

Why does Mrs. Hale pull out the messy stitches in Trifles?

In the one-act play Trifles by Susan Glaspell, a group of people come to a remote farmhouse to investigate the murder of John Wright, the owner of the farm. His wife, Mrs. Wright, is being held as a suspect. While Sheriff Peters, the county attorney, and Mr. Hale (who discovered the murder) walk around looking for clues, Mrs. Peters and Mrs. Hale wait in the kitchen. They talk about Mrs. Wright's loneliness and the deterioration of her joyful spirit after she married her husband.

While they wait, the women begin to find little clues suggesting not only Mrs. Wright's unhappiness but also possible abuse on the part of her husband. They come across pieces of quilt Mrs. Wright was sewing and notice that though most of the stitches in the quilt are "nice and even," the sewing on the last block is "all over the place." Perhaps Mrs. Wright was anxious and wasn't concentrating on what she was doing.

The women look at each other, as if suddenly aware that this could be an important indication of Mrs. Wright's state of mind. Mrs. Hale then begins to correct the sewing by pulling out the messy stitches and replacing them with neat ones. She says that bad sewing makes her fidgety, but the implication is that she is doing this to cover up Mrs. Wright's possible guilt. Although messy sewing would not at first seem a viable clue to the men, the women are able to use it to read Mrs. Wright's motivation for possibly committing the murder.

Later, the women notice the damaged bird cage and the dead canary in the box. They cover up this evidence as well. In their opinion, although Mrs. Wright may have killed her husband, it's possible that she was justified in doing so because of the way he had treated her. That's the real reason why Mrs. Hale pulls out and corrects the sewing, and also why Mrs. Peters hides the dead bird and does not show it to the men.

Mrs. Hale takes the stitches out of the quilt because their hasty stitching shows Mrs. Wright was preoccupied and possibly indicates her guilt.

To the women, the quilt provides insight into Mrs. Wright’s mind. The men would not notice this, but women would. To the men, all the little domestic indicators are meaningless trifles. The women realize their true meaning. They understand Mrs. Wright was miserable.

When Mrs. Hale sees the quilt Mrs. Wright was working on, she notices the stitching is off.

MRS HALE: (examining another block) Mrs Peters, look at this one. Here, this is the one she was working on, and look at the sewing! All the rest of it has been so nice and even. And look at this! It's all over the place! Why, it looks as if she didn't know what she was about!

Again, the men would not notice this at first. Upon closer examination, however, it would be evidence showing Mrs. Wright’s state of mind. Her husband was dead and her stitches were crazy. Even a man could figure out this meant Mrs. Wright was not entirely well and likely killed her husband.

Mrs. Hale feels sorry for Mrs. Wright. The whole situation is becoming apparent to her, from how Mr. Wright treated his wife to the circumstances of his death. She pulls out the stitches.

MRS PETERS: Oh, what are you doing, Mrs. Hale?

MRS HALE: (mildly) Just pulling out a stitch or two that's not sewed very good. (threading a needle) Bad sewing always made me fidgety.

No one noticed Minnie Wright withdraw from society and stop being her sunny self after she married. The women realize they didn't see the signs and intervene. There is a certain sense of guilt there, but also some womanly camaraderie in their instinct to protect Mrs. Wright by hiding evidence.

Why does Mrs. Hale remove the erratic stitches in Trifles?

Mrs. Hale's action of mending the erratic stitching in the play Trifles can be perceived from a number of angles.

One potential reason is that Mrs. Hale is trying to conceal the way that the stitching looks because it gives away the state of mind of Minnie Wright. If the men see the level of anxiety that Minnie apparently lived under, they would have all the more reason to accuse her.

MRS HALE I'll just finish up this end. [Suddenly stopping and leaning forward.]
Mrs. Peters?

MRS. PETERS: Yes, Mrs. Hale?
MRS. HALE: What do you suppose she was so nervous about

Another reason may be that the stitching represents the one part in the country wife's life that lets her shine. The stitching requires talent and creativity; these are luxuries that can seldom be performed particularly in the busy duties that come with tending farms. It is possible that Mrs. Hale simply wanted to keep Minnie Wright's only source of feminine ability looking the way that it is intended to look; this would symbolically retain some dignity for a woman that has been accused of a horrible crime.

[Mildly.]
Just pulling out a stitch or two that's not sewed very good. [Threading a needle.]
Bad sewing always made me fidgety.

Out of these two potential reasons, it all points out that Mrs. Hale is trying to conceal from the men the reality of Minnie Foster; these are not the days when women would be given the choice of claiming temporary insanity, nor battered woman syndrome which are the precise two things that Minnie was suffering from. In a rural, old-fashioned county, Minnie would be tried at face value no matter if she had a perfectly explainable reason to do what she did to her husband. Hence, by concealing any possible piece of evidence, Mrs. Hale would have spared Minnie another "giveaway" of her state of mind. The evidence comes with the way in which Mrs. Peters responds to what Mrs. Hale is doing

MRS. PETERS: [Nervously.]
I don't think we ought to touch things.

Mrs. Peters understands perfectly the reason behind the mending but, as a woman, she puts aside the fact that she is the sherriff's wife and, instead, she takes the side of Minnie because she understands that the woman had been, indeed, the victim of cruel and ongoing spousal abuse.