I had uterine atony after c-section then pph what causes it ?

Uterine atony: It can just happen but is associated with prolonged labor (tired uterus), distended uterus (large baby, increased fluid) and is related to number of previous pregnancies.

Lots of reasons : Atony is when the uterus doesn't contract back down after delivery and hemorrhage may result. It can occur for many reasons; prolonged labor, large baby and infection are just a few. It can even occur without any risk factors. They are medications to help but in extreme situations, hysterectomy may need to be done to save the mother's life.