Their are many causes of premature birth. ANY RISK FACTORS OR HIGH RISK PREGNANCIES CAN RESULT IN PRETERM DELIVERY.
It's very important that pregnancies with high risk factors are managed and closely followed up by the specialists obstetricians.
Other contributing factors are too young (below 18), too old (above 35), hypertension, diabetes, heart or kidney problems in mother, infection, leakage of amniotic fluid, infections like vaginitis, incompetent cervix, sudden accidents (trauma), bleeding in the placenta, if their are problems with the baby in the womb e.t.c