Final answer:
Capacitance after-firing of a spark plug is caused by the buildup of charge on the electrodes.
Step-by-step explanation:
Capacitance after-firing of a spark plug is caused by the buildup of charge on the electrodes of the spark plug. When the spark plug is connected to a voltage source, such as a battery, a potential difference is created between the electrodes. This potential difference causes charge to accumulate on the electrodes, creating a capacitance.