When one snaps the throttle shut, the mixture goes instantly rich for a split second. The flow through the pipe instantly stagnates as well. Therefore a vapour rich mixture is "very slowly" released out of the tailpipe where it hits oxygen and flashes off in pulses. This can continue as long as the injectors are still firing and the engine is coasting. It stops when the fuel is cut and the remaining fuel vapour in the pipe is expelled. Ultimately it all depends on the fuel-cut-off thresholds (ie above eg 2000rpm) and delay (for more than eg 2seconds).
Big backfires, even flames out of hihg-output engine exhaust after snapping the throttle shut from WOT is the same thing happening on a larger scale. Having just run very rich then snapping throttle closed, raw fuel residue may remain briefly in the combustion chamber or in exhaust path.
The fact that performance exhausts are rarely baffled and the pipe diameter is larger means the exhaust gas velocity is slower at all volumes, especially at closed throttle.
Some newer cars without "aggressive fuel cut off" keep the idle fuel going and supplement it with a bit more idle air, causing the car to coast (annoyingly) for a while, all in the name of smoothness and/or maintaining stoich without cutting injectors too often.
