If you shoot and pierce a vessel that is pressurised, especially if it’s near its pressure limit anyway, it will explode. Even if it’s just filled with an inert gas.

Actually even just making a hole by any means will significantly increase the chances of an explosion. This because the smallest crack can start a chain reaction where the material at the leading edge of the crack is bearing too much stress and the bonds break which then passes the stress on to the material straight after that. This all happens very quickly and can even accelerate as the hole grows bigger and gas starts moving.

It’s very lucky that the crack that formed didn’t propagate like that. It could be for many reasons though, like the crack front could have run into thicker material or a weld line or something like that and if luckily stopped the crack.