Often a newly-installed exhaust pipe or slip-on doesn't fit-up properly. Aftermarket pipes are not a precision fit and often they let in combustion air that, when mixed with unburned fuel, results in the fuel being burned in the muffler. This will result in backfiring. I suggest using high temperature silicon sealant at the joints to keep the air out.
I recommend Permatex Ultra-Copper high temp RTV silicone gasket maker #101BR for the aftermarket slip-on system joints. Good to 700˚F intermittent. Available in auto parts stores.