They suffer from these forms of severe Parental Alienation because their alienating parent is a Stage 3 Obsessed Alienator:
False Inducement of Fear
Dr. William Bernet explained that DSM editors can select one of three ways to include parental alienation in the upcoming 2013 edition-as a mental disorder, relational problem or as part of the appendix for further study. "Inclusion as a relational problem or as part of the appendix has not been discussed yet.". Bernet is the primary author of Parental Alienation, DSM-5 and ICD-11-the published version of the proposal Bernet and 70 other contributing professionals submitted to the DSM Review Committee at the Parental Alienation Conference in New York City 2010
Here is the website APA DSM-5 that has updates on progress being made for the 5th Edition, May 2013 release.