It isn't always caused by severe and prolonged trauma. It can be caused by particular types of trauma at a particular stage of development. That is, a trauma that interrupts the integration of the personality (such as a trauma that causes an unresolvable conflict with primary caregivers) at the developmental phase where the separate aspects of personality development are integrating. Typically the traumas are abuse related. But it is becoming more and more understood that the trauma doesn't have to be extreme or prolonged over many years as was once thought.
It's like fetal alcohol syndrome: It used to be believed that children with FAS were born to mothers who were heavy alcohol users throughout pregnancy. But now they know that FAS is caused by ANY amount of alcohol that the mother drinks at a very particular stage of pregnancy (it is something like the 21st or 28th day, but I can't recall which). So, a heavy drinker may drink every single other day of her pregnancy and have a child who does not have FAS, and another woman who has only ONE glass of wine on that particular crucial day may have a child who suffers the life long disability of FAS.
Same thing with DID - at that crucial integrative phase of personality development, a trauma that causes unresolvable conflict within the child - whether that be extreme sexual trauma or in attachment with the primary caregivers or something else - may cause DID.
Others believe DID is only caused by extreme trauma. Current research is discovering that is not the case.