Although the phrase "this is where I leave you" never appears directly in the text, it's an apt phrase for more than one of the novel's situations:
You could see it as Jen telling Judd that she's leaving him
You could even see it as Judd telling her the same thing.
You could see it as Mort leaving the family and forcing them to manage their problems by themselves.
You could even say that it's Tracy talking to Phillip, or Wade talking to Jen, or Wendy talking to Horry, or...
The point is that This Is Where I Leave You is about people at crossroads in their lives. Each of these characters is faced with the loss—on some level—of someone close to them. Their challenge is to pick up the pieces and learn how to live on their own.