Why does a 30 round magazine come while firing on a sks rifle?

Answer 1

If you meant come loose, it is because the rifle was not meant to use a 30 round magazine, and most were not meant to use a detachable mag. The standard is an attached, 10 round magazine. It is likely that yours fits poorly, feeds poorly, and will have a tendency to pop loose. Just my opinion, you are much better off with the original, since it works, and does not fall off the rifle.