So, assuming you have no misfires under load, when you back out suddenly on the throttle, the sudden fall in intake manifold pressure as the throttle closes, pulls the fuel puddle mass off the walls of the intake manifold, this transient “extra” fuel gets pulled through the engine, resulting in a rich mixture in the exhaust system. In conjunction, the spark mapping almost certainly retards the ignition timing, resulting in higher exhaust gas temperature. As this flow of rich, hot gas meets areas of higher oxygen content it can ignite, and burn. Sometimes (at low loads/temp) this is a gentle sort of “whumpf” kinda burn, at higher loads (like you get on a racing car for example) you get the full “bang” as the mixture explodes much more violently. As at idle, your exhaust system volume is large there will be plenty of trapped pockets of “fresh” air, to cause this, especially if the engine transitions into DFSO during the run down (and in effect is just pumping clean air into the exhaust). Of course, you may have an exhaust air leak as well.
If you have a scan tool, pull up the long and short term fuel adaption values and see where these sit.