It may be your tune. I found this response to a post from someone asking basically the same question:
So, the popping you are hearing is not back-firing. It is a popping resonating sound generated by a lean condition on decel. When you suddenly close the throttle, the incoming air is choked off and the closed throttle also causes the bike to return to idle on the very lean idle circuit of the carb(s). However, momentum of the bike is actually "driving" the engine in a sense, and that engine is still spinning at rpm that needs far more gas than the closed throttle/carb can provide. The engine is still spinning but the incoming fuel rate is very low, until just enough builds up in the combustion chamber to "fire" on the exhaust stroke of the engine.
To know this for sure, the next time it happens, pull in the clutch and the popping will stop immediately, because the bike's momentum is not pushing the engine to spin. Don't "change the plugs" like some people suggest, its a waste of time and money. Messing with the ignition timing really won't help either, because it is still a lean fuel/air mix on decel that is the cause. Note, I said cause and not problem, because the popping is not at all damaging to the bike. So don't expect your dealer to fix the "problem" under warranty because it is not a warrantable situation. The EPA drove this more than anything else.
A recent update on this. Ever wonder why the stock mufflers on your bike are so big and heavy? Well, one reason is sound attenuation. Big mufflers provide a lot of volume for the rumbles and popping in the exhaust to be toned down by resonating in the larger muffler canister. Motorcycle builders also have to meet sound regulations along with emission standards. Some of that sound proofing is in the mufflers. But aftermarket mufflers, easier breathing, usually lighter and smaller, don't offer that same volume. Less volume, less sound attenuation = more noise and more popping. Again, don't complain to the dealer about this unless you plan to spend the money to correct the fuel/air mix ratio to make up for the increased airflow through the engine.
Modifying the exhaust (especially if it gets louder) will only make it worse, and louder when it pops. Because most aftermarket pipes produce "lower back pressure", it makes matters worse yet because lower back pressure pipes actually cause the fuel/air mix to LEAN OUT EVEN MORE. The solution is in the carbs, and it requires a richer fuel/air mixture. The only logical way to do that is spend about $300 to get a Dynojet or K&N brand "jet kit" installed. Do not mess with that carb jets yourself, do not mess with airbox, do not simply install a high flow air filter (which also can lean out the mixture) itself. If you want the bike to run a lot better and no pop on decel, spend the bucks on a jet-kit, you will be very happy with the results and it'll do a LOT more for performance than louder pipes ever will. In fact, I'd be willing to bet a jet-kitted bike with stock pipes would probably beat a stock carbed bike with aftermarket pipes.