NOT KNOWN... No one knows the final "trigger-pull" that causes lupus. We do know genetics play some role. We do know that hormones play a major role (reason for female dominance and females rarely getting sle before puberty). But why some with similar factors get disease is not known at present.
Genes, environment. This answer will change as we know more about lupus. We believe that for a person to get lupus, from the limited genetic information we have now, about 10% is from known genetic causes and remaining 90% unknown genetic causes and environmental causes. Environmental causes are largely undefined but important. Think about this: only 30% of identical twins develop lupus together.