In brief, under the pressures and self doubts caused by a divorce, a parent with unresolved childhood family relationship trauma that left them acutely vulnerable to fears of inadequacy or abandonment can have these fears re-triggered by the divorce.
To survive their crushing emotions, the alienating parents uses 3 psychological defense mechanisms:
1.) delusion: the alienating parent creates a self delusion that the other parent is worthy of rejection and abandonment
2.) splitting: thinking their ex to be all bad
3.) projection: imagining that every personal negative feeling is a thing done by their ex. For example, my feelings of abandonment are really my ex's feelings of actually being abandoned.
They can say to themselves, "See, I am not the abandoned, unworthy parent. My ex is".
To support their self delusion, they manipulate their child to reject/abandon the other parent, thus triangulating them into the marital conflict. Once the child claims (false) victim hood status, most people and most therapists naively make the mistake of piling on against the rejected parent, therefore validating the delusion of the alienating parent, which spreads to the child
Said in another way, as excerpted from Foundations: "At its foundational core, parental alienation represents the reenactment of a false drama of abuse and victimization from the childhood of a narcissistic/borderline parent that is embedded in the internal working models of the "alienating" parent's attachment networks. This false drama of a the reenactment narrative is created by the the psychopathology of a narcissistic/borderline parent in response to the psychological stress of the divorce and the reactivation of attachment trauma networks as a consequence of the divorce experience. In truth, there is no victimized child, there is no abusive parent, and there is no protective parent. It is a false drama, an echo of a childhood trauma from long ago"