As the story unfolds, it becomes more and more clear that the Wright household was not a happy one. While the men look for a clear, rational cause of murder such as theft, the women discuss Minnie's emotional state and how she must feel concerning the murder; they see evidence of strife in seemingly unimportant things like Minnie's sewing. The reveal comes when they find her dead canary in a box; the women realize that Minnie felt trapped in a hostile marriage and her husband killing her canary was the last straw:
MRS. HALE: [With a slow look around her.] I wonder how it would seem never to have had any children around. [Pause.]No, Wright wouldn't like the bird -- a thing that sang. She used to sing. He killed that, too.
(Glaspell, Trifles, etext.virginia.edu)
This reveal shows why Minnie's sewing had started to suffer, and why the rational motive for the murder is not obvious to the men. They can't see past their own prejudices, and don't think the emotional health of a woman is important. Clearly, though, the slow abrasion of abuse was enough to make Minnie feel that she had no other choice; she doesn't even care if she is convicted as long as she never has to deal with her husband again. The absence of the canary means nothing to the men, but is the key component in understanding the story for the women and the audience.