What causes a placental abruption?

Shearing forces. Abruption is a bleed from the placenta; a disruption of either the mother or baby's blood supply. Many causes: trauma, high blood pressure, drugs (cocaine), preterm labor, polyhydramnios. Can be very large or very small, so even US can miss a diagnosis. Symptoms: usually preterm contractions, sometimes vaginal bleed. Fetal monitoring is necessary. Wait, watch; abruption can self-heal sometimes.
Various causes. Most placental abruptions have no obvious cause. It is known that tobacco abuse, drug abuse, trauma, intruterine infection, and a history of previous placental abruption all will increase the risk for a placental abruption. Labor itself can lead to an abruption as well.