When does an Engine "Pop"

Generally, An engine pops when it is improperly tuned.

More specifically, in what is often called spitting, backfiring, or coughing, these would have happened

1. A spark plug "sparking" when it isn't its turn and the exhaust valve is open. Suppose there is air/fuel mixture that is too rich in tje engine cylinder and there is unburned fuel in the exhaust system, cross firing from one spark plug wire to another can occur if they are touching each other. When this happens while the exhaust valve is open, it will ignite the rich / unburned mixture in the exhaust manifold and tail pipes and result in a big bang.
2. Not correct valve timing, esp timing that is too late [etarded] can cause this same thing as above in addition to an engine that is running too rich. When the exhaust valve opens, the mixture isn't done burning so it ignites the unburned fuel in the exhaust system and causes a loud bang.
3. A faulty distributor sends causes a spark to be done by a plug that isn't ready for it yet
4. The air/ fuel mixture is too lean, esp in the morning . Backfiring will happen thru the Carb
5. A bad ignition system eg cross firing. The pop will come out with a flame from the exhaust