My Daughter Had A Baby Recently. Her Baby Progressed Down

My daughter had a baby recently. Her baby progressed down the birth canal to the point that with manual manipulation the baby's head was showing . She had pushed for 3 hours, baby was showing fetal distress. A c-section was performed. The baby was fine but the mother had two hours of surgery to fix the shredded uterus, and .cervix. What causes the uterus to shred and how can the cervix be injured?