Efi + CNG=Backfire (sooner or later)
Reason could be anything from MAP sensor faliure to weak ingnition system to disturbed air/fuel mixture. It could even be due to weak engine valves, which is caused by CNG.
I've seen Corolla, City, Santro and even Parado backfire on CNG causing air intake manifold to either get off or break apart, and if the backfire is intense it could even ruin the MAP sensor.