What causes a cholinergic crisis?

The most common cause of the cholinergic crisis is from medication or accidental exposure to insecticides such as Malathion or herbicides.
Other sources include nerve gas such as sarin gas and VX gas. Use of these weapons is against the rules of warfare set by the Geneva Convention established in 1925. Recently they have been used as weapons of terrorism.
A cholinergic crisis describes the condition of excess acetylcholine at neuromuscular junctions and the synaptic gap in the central and peripheral nervous system.
Acetylcholine acts as a neurotransmitter for muscle fibers, in the peripheral nervous system, and extensively in the brain. The enzyme acetylcholinesterase rapidly breaks down the neurotransmitter once it is released.
Not enough acetylcholinesterase leads to a buildup of acetylcholine and an increase in the level and duration of neurotransmitter signal. Symptoms of the cholinergic crisis include excess salivation (drooling), lacrimation (many tears), emesis (vomiting), and diarrhea.
Striated and smooth muscles eventually stop responding due to the accumulation of acetylcholine. The result is paralysis and respiratory failure.