The answer to this question can be found in the chapter entitled "Snapshots." Up until this stage, Abigail is a woman who is presented as being haunted by her dead daughter and increasingly alienated by her husband's inability to let Susie go. She needs an escape from this claustraphobic situation, and so first tries adultery with Len. However, it is clear that this is not enough of an escape for her. Note how the idea of going to California came about, and why it was so attractive for Abigail:
My mother made it through only one winter in New Hampshire before she got the idea of driving all the way to California. It was something she had always thought she would do but had never done. A man she met in New Hampshire had told her about the work to be had in wineries in the valleys above San Fransisco. It was easy to get, it was physical, and it could be, if you wanted it to be, very anonymous. All three sounded good to her.
Thus we can see that Abigail went to California because it was something that she had always thought about doing before the onset of family life. In addition, she could easily pick up physical work, to help her forget her worries, and she could remain anonymous, and avoid the kind of questions that would prevent her from truly escaping.