Why does Oxy Acet crack sometimes?

Done a little MIG welding, and Arc, but never Oxy Acetylene. Had a go with some gear my friend has, and found that I often get a laud CRACK and this occasionally blows out the flame, its very similar to a small explosion.

why is this?

Is it because I am holding the flame too close to the weld?

Or is it to do with the mix of the gasses?

Any help offered most welcome.

Thanks all Deb
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