Just out of curiosity, what causes an engine to backfire? My truck never has but my lawnmower does it occasionally and scares the outta me.

 

Engine runs lean and/or hot, backfire from intake/carb. Engine runs rich, unburned fuel ignites in the exhaust and causes backfire on shutdown. Wrong ignition timing, bad gas, exhaust leak at manifold/muffler, lots of things. Mostly caused by wrong fuel mixture and air/vacuum leaks.