What causes babies to end up in the breech position?

Many factors. Causes for breech presentation include preterm babies, twins, polyhydramnios (excessive amniotic fluid), fibroids, uterine anomalies, or placenta previa. Many breech presentations have none of these risk factors.
Usually no reason. Twin pregnancies can be breech because of limited room in the uterus. Some babies may have a larger head than usual that does not fit easily in the mother's pelvis. Sometimes certain abnormalities of mother's pelvis or uterus may force baby to assume a breech position. Most cases there is no obvious reason.