After the Philistines captured the Ark of God, they took it from the battleground at Ebenezer to the city of Ashdod.
They carried the Ark of God into the temple of Dagon and placed it beside the idol of Dagon.
But when the citizens of Ashdod went to see it the next morning, Dagon had fallen with his face to the ground in front of the Ark of the LORD! So they set the idol up again.
But the next morning the same thing happened -- the idol had fallen face down before the Ark of the LORD again. This time his head and hands had broken off and were lying in the doorway. Only the trunk of his body was left intact.
That is why to this day neither the priests of Dagon nor anyone who enters the temple of Dagon will step on its threshold.