Why do I have a pop when I'm turning off a acetylene torch?
You're doing it the right way, oxygen off first. When you turn the acetylene off a little gas remains in the pipework after the valve is shut, hence the remaining little flame. Quite normal.
Try reducing both gasses in stages so you end up with a smaller quiet flame. You may need to do it in 3 stages so you don't have large acetylene flame and smuts floating everywhere. Then when you do turn off the oxygen the pressure will be so low anyway it shouldn't do it at all. I hate garages that always shut off the acetylene first with a loud crack - Good job they have flashback arrestors.
its normal ..but it does that to everybody