Final answer:
The second most common cause of postpartum hemorrhage after uterine atony is retained placenta, which can lead to significant bleeding and requires prompt treatment.
Step-by-step explanation:
The second most common cause of postpartum hemorrhage (PPH) after uterine atony is retained placenta and placental tissue. When the placenta does not completely detach and expel from the uterus after childbirth, it can lead to significant bleeding. Apart from retained placenta, other causes include genital tract trauma, coagulopathy, and uterine inversion. Recognizing and treating the underlying cause swiftly is critical for the well-being of the mother.