You need a spark arrestor. The above statement is true if one of the exhaust manifolds or mufflers is cold or cooler than the other.
Unburnt gas igniting in muffs. Bad unclean carbs is also a possibility. My old quad used to do that to I think that when you let off the gas the rpm's are still kind of high and more air is getting to the engine and less gas and it kind of backfires sort of like but its not a bad thing I might have it backwards it could be running rich to.