What causes a baby to be born at 25 weeks?

Sometimes necessary. In rare cases, the expectant woman is very ill from preeclampsia, maternal condition, etc or is in unstoppable labor or at risk from infection or bleeding, so needs delivery to preserve the health of her and often also the baby. In other cases, the condition of the fetus is deteriorating so delivery is carried out to prevent further harm or death. This is a very difficult decision by expert team.
Many things. Birth at 25 weeks may be the result of preterm labor or surgical delivery. Infection in the womb is associated with preterm labor, but there are many other possible associations as well. Cesarean delivery at 25 weeks is typically for the health of the mother; most commonly to resolve maternal preeclampsia. Rarely, it is for the health of the baby due to placental or other problems.