Normally, the round doesn't arm until it is clear of the tube. Contact fuses react to sudden changes in acceleration (like hitting the ground) to trigger the boom. Normally, this arming is set by the force of launching and the round is well clear of the tube before it is dangerous.

This guy managed to somehow arm a round *before* it was dropped down the tube. So when the launching charge went off, the fuse detected the sudden change of speed, and decided "time to blow up."

For this to happen, they would have had to remove the safety *before* running around and setting up to fire. That's asking for trouble.