Since her arrival in Egypt and her marriage to her husband, the protagonist has suffered a massive change in her life. At first, the mutual love of her relationship with her husband carried her through the difficulties of adapting to a different culture and land. However, as the narrator's account makes clear, her husband slipped out of love with her gradually, leaving her trapped in Egypt with a daughter that she loves, meaning that she is unable to leave. Note how the narrator describes this process of losing her lover:
I watched him vanish--well, not vanish, slip away, recede. He did not want to go. He did not go quietly. He asked me to hold him, but he couldn't tell me how. A fairy godmother, robbed for an instant of our belief in her magic, turns into a sad old woman, her wand into a useless stick.
The narrator goes on to describe how it was her "foreignness" that was a key factor in him falling out of love with her. This is the key change that occurred in the protagonist's life, which has left her stranded on the fringes of this culture. Of course, as the title suggests, her position is encapsulated in that of the sandpiper, the bird who dwells permanently on the fringes of the sea and land, belonging fully to neither.