94 wrx jap de-catted exhaust straight through to a hks drager exhaust box backfiring when changing gear and when accelerating and releasing to slow down it pops back usually one loud bang scares the pants of people in car parks any ideas what could cause this

 

I Don't know what causes it.
I Don't care
All I know is mine does it aswell and I love it

 

Most likely unburnt fuel igniting in the hot exhaust on overrun, assuming that your car is driving OK at the time.

People pay big money to get that effect!