my understanding (which i got from Paul A.) is something like
the three things it takes for fire (backfire too) are fuel,
heat and air..
there is always unburnt fuel coming out the exhaust
there is always heat coming out the exhaust..
if there is an AIR leak (expecially at the joint between the
header and the manifold) you have the condtions needed to make
FIRE! .. fix the leak, the backfire goes away.. ..
in my own case i had a "burble" on compression braking..
retightened, refit that clamp thingy and the burble went away!