It was always called hydramnios, but for reasons best known to the medical fraternity is called polyhydramnios.
Too much fluid in the amniotic sac....... around the baby.
The baby drinks the fluid and makes the fluid.....so any abnormality of this mechanism in the baby will cause hydramnios.
Now after saying that...more than 50%+ cases of hydramnios, no cause or abnormality is found in baby or mother.
daffy's link will give you the rest of the causes of hydramnios.