Because the air pressure inside the balloon is very high, when it is pierced with a hole, all the gas is squeezed out from this small hole. The balloon itself is thin and brittle, so it can't stand such a big impulse. At the moment when the gas was squeezed out, it was torn apart and made a huge noise. In our opinion, this is an explosion.

It's easy to keep it from exploding. Make a balloon with good material and good pressure resistance. ...