Do not lift up your horns on high; speak not with a stiff neck. For not from the east or from the west and not from the wilderness comes exaltation. When Belshazzar saw the handwriting on the wall (Daniel 5), he wondered if rebellion against him had arisen and sent for all the wise men of his kingdom, but none could interpret the writing. He then realized that his downfall was from God, who humbles one and exalts another, as it is said, "God brings low and he raises up" (1 Samuel 2:7).
Another explanation: "This one will be humbled" refers to the incident with the Golden Calf when the people said of Moses, "This fellow Moses, we don't know what's become of him" (Exodus 32:1). "And this one will be exalted" refers to the time when God said, "Whoever has sinned against me, him will I blot out of my book" (Exodus 32:33), whereupon the Levites came forward and slew the transgressors.