Cholly Breedlove’s rape of his daughter, Pecola Breedlove, instigates her downfall because it takes away her voice. During the graphic assault scene, Morrison notes the “silence of her stunned throat.” It’s reasonable to claim that the traumatic sexual abuse produces Pecola’s inability to stick up for herself. The rape has rendered her passive and made her feel like she has no choice but to except other people’s mistreatment, whether it be from the man at the grocery store, Maureen, her mom, or her dad.
Another way that the rape brings about her downfall is that it alienates her. After the rape, Morrison describes Pecola as lying on the kitchen floor and "trying to connect the pain between her legs with the face of her mother looming over her.” The specific derangement connects to a general detachment that follows Pecola to her death. Since her Black identity has been the site of such much pain and abuse, it’s possible to argue that the rape is what causes Pecola to idolize white people and wish that she had blue eyes.
For a third piece of textual evidence that reveals how Cholly’s rape instigates Pecola’s downfall, think about how grownups in Pecola’s community react to Pecola’s situation and how their responses make matters worse. One might also think about how the phrasing of the question reinforces sexist tropes about "fallen women." It’d probably more mindful to say that Cholly’s rape causes Pecola deep suffering and trauma.