A 27-year-old G5P4 at 36 weeks with transverse lie and polyhydramnios (AFI 28 cm) is admitted for elective caesarean section. Thirty minutes after admission, she experiences spontaneous rupture of membranes with a large gush of clear fluid followed by sudden, prolonged fetal bradycardia (60 bpm). Vaginal examination reveals a pulsatile loop of umbilical cord in the vagina below the fetal shoulder, as shown by structure **A** in the diagram. The resident immediately places the patient in knee-chest position and manually elevates the presenting part. What is the PRIMARY PATHOPHYSIOLOGICAL MECHANISM of fetal compromise in the condition marked **A**?