PAS is a kind of brainwashing. If the parent with custody can convince the child that he or she doesn’t want anything to do with the father, that carries weight in court, since one of the factors taken into account regarding contact is the wishes of the child. For obvious reasons, it works better with younger children, who are more easily swayed and subject to emotional pressures. In difficult situations, they naturally want to please the parent they’re with.
It can happen for any number of reasons, from fear of losing the children to the other parent to revenge. Sometimes trying to establish the root cause can be impossible.