How would childhood sexual abuse cause psychosis?

Depends on the abuse: I have worked w/ 1000's of individuals who were abused as children. The range of outcomes is very diverse, depending on the type of abuse, the number of abusers, the severity of abuse (painfulness, intrusiveness, whether done under threats), innate resourcefulness of the victim, social support network, etc. Some children were so overwhelmed by the abuse they developed multiple personality dx.

Disrupted regulation: The cause in these instances is hard to establish. However, intolerable emotional states during or following disruptive experiences, are handled by the child the best way he or she can come up with. These ways of handling painful realities can become ingrained and are frequently magical. In the adult this unrealistic way of handling realities can look defensive, dissociative or psychotic.

Abuse is a Risk: Childhood sexual abuse and severe psychological trauma (depression, anxiety, substance abuse) have all been linked to later psychotic disorders. Studies examining subjects (12 to 16 years) who were sexually abused with penetration evidenced a 3.4 percent probability of psychosis (archives of general psychiatry).