Why has the narrator returned east?
The narrator says that she owes her existence to her mother "three times." The first time is when her mother saves herself (and her unborn child) during the trapeze accident. The child eventually is stillborn. But the narrator adds that if her mother would have tried to save Harry, she may have lost her own life. In saving herself, Anna (the mother) stays alive and can therefore give birth to the narrator at a later time. The second time is when Anna meets her second husband in the hospital. This husband is the narrator's father. Their meeting and falling in love leads to the narrator's birth. The final time is when Anna saves her daughter from a house fire. This is the "leap" from which the title is taken. These events are significant because they show how the narrator feels indebted and dedicated to her mother.
That being said, the narrator will do anything to help and/or repay her mother. The narrator had been living in the west. And apparently she feels that her life, or recent life in the west, has been a failure. This probably makes the decision to return a bit easier. When her father dies and her mother starts to go blind, the decision is made. She returns east to care for her mother. Particularly, she returns to read to her mother. When Anna was in the hospital, she learned to read, and the narrator refers to this as exchanging "one form of flying for another." In other words, she trades acrobatics for reading. On why she returns east, the narrator says:
Since my father's recent death, there is no one to read to her, which is why I returned, in fact, from my failed life where the land is flat. I came home to read to my mother, to read out loud, read long into the dark if I must, to read all night.