Why do cars back fire?

Wrong timing or dirty plugs.

Can be a multitude of causes usually due to incomplete combustion from an unbalanced air/fuel mix, poor spark delivery or lack of compression resulting in a slight miss or misfire which sends unburned fuel to "pop off" in your exhaust. Some is to be expected and normal, for example a sudden change in throttle position, like decelerating but it should not be routine. Start by checking for spark, have codes read if CEL is on and then move on to compression test if that looks good. If it's cylinder specific (code points to specific cylinder) , Check plug and wire on that cylinder/ Check plug gap and if any are fouled. Pull them one at a time if you have to to isolate cylinder (use insulated tool) Good luck.