That's not unusual. Heavy acceleration will have the carb flowing maximum fuel to the cylinder. When you let off the gas, there will be residual fuel going to the hot cylinder. Even though you've let off the gas, the left over fuel going into the engine will be ignited by the hot cylinder and compression rather than the spark. That's why it backfires because the ignition is off time with the valves. The same effect can be done in a car by shutting off the engine at speed. If it's back firing while you're on the gas, that a different story.