Sometimes that's how traumatic memories surface. You sometimes remember the "safest" part first and then as you body/mind allow, more of the memories become visible. You repressed the memories as a coping skill and they will surface when you are ready to heal. Do you have a therapist? Talking through what you do remember is a good way to get started and a good way to learn healthy coping skills for when the memories fully emerge. Some people never remember everything - again, for self-protection.