A 42-year-old male farmer from Punjab is brought to the emergency department 2 hours after accidental exposure to an organophosphate pesticide during crop spraying. He presents with profuse salivation, lacrimation, pinpoint pupils, muscle fasciculations, and respiratory distress. His wife reports he was conscious when found but rapidly deteriorated. On examination: BP 88/54 mmHg, HR 52/min, RR 28/min, SpO₂ 85% on room air. Pupils are 2 mm bilaterally. Which of the following best explains the mechanism of toxicity in this case?