Backfire/Pop is semi normal but dangerous.

It is caused by rich mixtures but the reason it "pop's" upon releasing throttle is coz your A/F suddenly goes lean and ignites the rich unburnt mixture in your exhaust pipes.

If it happens often you really should upgrade to a metal CAT if you don't already have one, otherwise you won't have much of a cat later when the ceramic shatters/melts and you get EPA'd for a 'catless' car....