Why does Sugar, from Walk Two Moons, decide to take a trip to Idaho?
Because Walk Two Moons is written from Sal's perspective, we only have what Sal knows to go off of, so it's impossible to know the exact reason why Sal's mother, Chanhassen or Sugar, decided to take her trip to Idaho. However, we can piece together a pretty good idea from Sal's memories and observations of Sugar.
Before she leaves, Sugar tells her family that she needs to leave to "clear her head and to clear her heart of all the bad things" and to "learn about what she was." She tells her family that this is something she has to do on her own. She specifically chooses to go to Idaho because she has a cousin there who she hasn't seen in 15 years. Sugar thinks that since her cousin only knows her from the past, her cousin will be able to tell her what she's really like, and who Chanhassen truly is, aside from being a mother and wife.
Those are the reasons that are clearly stated for Sugar's trip in Walk Two Moons, but there are other factors that play into the story as well. We find out that Sugar wanted to fill their home with children and also that she lost a pregnancy and had to undergo a hysterectomy, which Sal blames herself for. Losing a child is always a tragedy and for someone who wanted a large family, the hysterectomy at the same time is an added blow. This, combined with Sal's memories of her mother wandering the fields, her distraction, and her decision to cut her hair out of the blue are clues to Sugar's depression.
Something that is repeated throughout the book is Sugar feeling that she's not good compared to the Hiddles. She tells Sal's father before she leaves that she feels "rotten in comparison." Sal says that it seems like Sugar wishes her husband were meaner and less good. Sometimes when people are depressed, they don't feel that they deserve kindness. In Sugar's case, her husband's kindness and "goodness" serve as a constant reminder that she isn't good and she can no longer provide the large family they dreamed of. All of this factors into the reason why Sugar feels the need to get away and clear her head on her trip to Idaho.