If you have aftermarket or modified mufflers then it is probably caused by the PAIR valves introducing air into the exhaust.
If you have OEM mufflers (unmodified) then it's probably a leak in the exhaust where the mufflers join the headers, or else the balance pipe under the engine has a hole in it. Either way, air is getting into the exhaust and burning the unburnt gas in the hot exhaust on deceleration.