exploding back box*misiles away!!*

hi can any one explain why when i put the center pipe back in to my fmf back box (the smaller pipe in the middle) of the back box to reduce noise,i rode for about 2 miles on road then about 2 mile green laneing-the bike suddenly started to back fire,i thought water had got in so revved the crap out of it , the reves were low, it eventually died and to my surprise the back box had been turned into a second combustion chamber blowing out the exit pipe completely -least to say its fucked the back of my new bike ! bought it Monday,any ideas-.could it be the ignition coil assembly?it has a yz top end and wr gearbox