340challconvert nailed it. When the throttle is released, vacuum skyrockets and pulls fuel kicking and screaming through every carb circuit that'll provide it. The inrush of fresh oxygen (air) due to the sudden drop in exhaust system pressure is all the hot, unburned fuel needs to ignite. It's part of the territory with high-perf carbureted engines.
When this happens in a EFI car--particularly one with a custom tune--it makes me laugh. Guys will talk about how awesome their tune is, then it pops and bangs like crazy when they let off. That means their tuning guru paid absolutely no attention to DFCO (deceleration fuel cut-off) or dashpot. There's a perfect example of this right around the corner from me. It's a C7 'Vette that sounds like the 4th of July on decel. I asked who tuned it so I'd know to avoid them later.
If you have an upstream exhaust leak, you'll get an outright backfire through the exhaust. My Trans Am's clutch linkage had worn a small hole in a header primary. After winding it out, I could make it shoot a single 6-8' blue flame out the LH tailpipe, accompanied by what sounded like a rifle shot, simply by compression braking.
