A 48-year-old man from Kolkata with a 2-year history of progressive weakness presents with difficulty climbing stairs and rising from a chair. He reports that weakness is worse in the morning and improves slightly with activity. On examination, he has proximal lower limb weakness (hip flexors 3/5, knee extensors 3/5) and diminished deep tendon reflexes. Serum calcium is 8.2 mg/dL (low-normal). Chest X-ray reveals a 3 cm opacity in the left hilum suspicious for malignancy. Repetitive nerve stimulation shows an incremental response (20% increase in CMAP amplitude after 10 seconds of 10 Hz stimulation). What is the primary mechanism of neuromuscular dysfunction in this patient?