What significance does Mama's memory of having been hooked in the side while milking a cow coincide with the climactic moment of the story?
Another possible similarity is that Mama has been unable to "let go" of Dee and how she has changed so much. She has been tethered to the hope that Dee might change (as evidenced in her daydream about Dee and her being on a talk show and reconciling); however, she realizes that this is an unrealistic hope, ultimately. At the end of the story, when Mama refuses to back down about the quilts, she has finally freed herself from Dee's grip on her. She can now be content with knowing that she cannot change Dee and that Dee will have to be the one to repair the relationship with her (Mama).
From the beginning of the story, it is clear that mama is an exceptionally strong women (physically and mentally). Being hooked by a cow emphasizes her strength; despite her injury, she did not become weak. In addition, the mama states (as narrator) that she loved to milk until the cow hooked, or hurt, her. At the climax of the story, when Wangero/Dee tries to take Maggie's quilts, not only does mama stand strong, her feelings for Wangero/Dee also change. She no longer accepts Dee's behavior without asking questions or resisting.