We need not suppose a complete shattering of the altar, but rather the appearance of a crack or fissure in the fabric, which, extending from top to bottom, caused the embers and the fragments of the victims to fall until they reached the ground.
Related Commentaries of 1 Kings 13:5
1 Kings 13:5
5 The altar also was rent, and the ashes poured out from the altar, according to the sign which the man of God had given by the word of the LORD.