**Core Concept**
Postpartum hemorrhage (PPH) is a leading cause of maternal morbidity and mortality worldwide. Secondary PPH, which occurs after the first 24 hours postpartum, has a different set of causes compared to primary PPH. Understanding the etiology of secondary PPH is crucial for its prevention and management.

**Why the Correct Answer is Right**
The most common cause of secondary PPH is retained products of conception (RPOC). This can occur due to incomplete uterine evacuation at the time of delivery, especially in cases of retained placental tissue or fragments. The presence of RPOC can lead to continued bleeding, infection, and other complications. The uterus may not respond adequately to oxytocin or other uterotonic agents due to the presence of RPOC, further exacerbating the bleeding.

**Why Each Wrong Option is Incorrect**
**Option A:** Uterine atony is more commonly associated with primary PPH, which occurs within the first 24 hours postpartum.
**Option B:** Coagulopathy can be a cause of PPH, but it is less common than RPOC as a cause of secondary PPH.
**Option C:** Placenta accreta spectrum disorders are more commonly associated with primary PPH, although they can occur in any trimester.

**Clinical Pearl / High-Yield Fact**
Retained products of conception should be suspected in any patient with secondary PPH, especially if there was incomplete uterine evacuation at the time of delivery or if there is a history of previous uterine surgery or trauma. A thorough examination and imaging studies (such as ultrasound or CT scan) may be necessary to diagnose and manage RPOC.

**Correct Answer:** D. Retained products of conception.