What reason other than "bad sewing" making her "fidgety" would compel Mrs. Hale to rip out the crazy stitches?
Mrs. Hale and Mrs. Peters have realized, at this point, that Mrs. Wright is responsible for her husband's death. The stitching being erratic is symbolic of Mrs. Wright's mental state (one of disarray) or it could be symbolic of Mrs. Wright's anger at her husband. Mrs. Hale rips out the stitching so that it cannot be used as evidence to show that Mrs. Wright was "crazy":
The women consider their alternatives: disclose what they know, or cover up the clues that suggest a motive to the crime. Mrs. Peters finds the answer in the men’s patronizing treatment. ‘‘My, it’s a good thing the men couldn’t hear us,’’ she says half-jokingly. ‘‘Wouldn’t they just laugh! Getting all stirred up over a little thing like a—dead canary.’’ Without admitting it aloud, this is the only excuse the women need to keep Minnie’s private agony a secret. (Enotes)