A 32-year-old multigravida at 34 weeks of gestation with chronic hypertension and superimposed pre-eclampsia presents with sudden-onset severe abdominal pain, moderate dark vaginal bleeding, and a woody-hard tender uterus. Cardiotocography shows prolonged decelerations with loss of variability. Bedside ultrasound reveals the structure marked **A** — an 8 × 5 cm ill-defined hypoechoic-to-mixed-echogenicity collection between the posterior placenta and myometrium, partially lifting the placenta. Laboratory findings show platelets 82 × 10⁹/L, fibrinogen 165 mg/dL, and D-dimer >20 μg/mL. What is the pathophysiological mechanism underlying the formation of the structure marked **A**?