How does organophosphate poisoning cause muscular paralysis?

Inhibits AChE: Organophosphates inhibit acetocholinesterase, which is responsible for recycling actetocholine. This leads to muscle overstimulation and rigid paralysis (amongst other things).

Acetylcholine: The fertilizer or pesticide chemicals can directly block the critical chemical acetylcholine which allows nerve muscle communication at the neuromuscular junction, and thus, create a life-threatening situation. Wear protective gear if using such sprays.