(15) Thus says the Lord GOD of hosts:
Verse 15 introduces a man named Shebna, called the “steward” who was “over the house,” that is, the royal household. The word for “steward” can also indicate the treasurer or the prefect of the palace, both pivotal positions. All indications point to Shebna being the man in highest authority under Hezekiah. He was essentially the king's right hand, not unlike Joseph in Egypt under the Pharaoh.
God gave Isaiah the task of delivering His judgment to Shebna, which began with removing him from office. After this, Scripture refers to him as “Shebna the scribe” instead of “Shebna the steward” (II Kings 18:18-37; 19:2; Isaiah 36:3-22; 37:2), having been given a position of lesser authority. The remainder of God's judgment was that he would be deported to another country—likely Assyria—where he would die.
God's charge against Shebna deals with his ostentation and presumption. He was not the king, yet he presumed to have a burial place among the royal dead, who were interred in sepulchers of prominence on a mountain. He tried to give himself greater honor than had been bestowed upon him—a bold move that indicates his mind's tendency. He was more interested in his own affairs and his place in history than he was in simply doing his job.
His “glorious chariots” of verse 18 illustrate a focus on image and reputation rather than on substance. He was more concerned about his own glory than in the well-being of the nation, which was crumbling around him. Because of his focus on himself instead of God's will, God took away his authority and later removed him from the land altogether.
— David C. Grabbe