What causes your water to break
During pregnancy your baby is floating in a bag of amniotic fluid. The bag is called your amniotic sac. During pregnancy the amniotic sac is attached to the walls of the uterus, kind of like they are glued together. During the end of the third trimester the amniotic sac will naturally start to detach from the walls of the uterus. As the amniotic sac and the uterus separate it will cause hormone to be released. These hormones will trigger you to start labor and will prepare your cervix for labor my ripening and softening it.
Once the amniotic sac and the uterus have separated, the amniotic sac will be more fragile and more easy to break. Some women will have their water spontaneously break with a big gush and other will have a leak. Some women will not having their water break and their doctor will manually do it.