Explain why one spark will ignite propane gas and no more sparks are needed for the propane gas to continue burning.

The spark starts a chain reaction

Explanation:

The spark combusts a few molecules of propane gas in the presence of oxygen. The reaction is exothermic. Therefore the dispensed heat energy spurs the combustion of the nearby propane molecule that emits more heat energy. This continues, very rapidly, causing an explosion due to the high amounts of energy released from the combustion reaction.