Many things. Mother nature can be tough. Every pregnancy poses risks to mom or baby. The goal of modern obstetrics and maternal-fetal medicine is to decrease the chances of big problems occurring. Uncomplicated pregnancies are more common than complicated ones in which mom or baby is at high risk for preterm birth, bleeding, hypertension, diabetes, multiples, birth defects or other threats to their life/health.
Take it case by case. Many patients worry that they are "high risk" because of common problems that really don't impact pregnancy such as history of early miscarriage in the past. It's important that you and your doctor discuss what your particular issues are and avoid the general term "high risk" since it means different things to different people. Most complications in labor happen in the "low risk" unexpected cases.