Answer: We can find our answer regarding the naked preaching of Isaiah in the book named after him.
In the year that Tartan came to Ashdod, when Sargon the king of Assyria sent him, and he fought against Ashdod and took it; At that time the Lord spoke by Isaiah the son of Amoz, saying, "Go and take the sackcloth off your loins, and take your shoe off your foot." And he did so, walking naked and barefoot (Isaiah 20:1 - 2, HBFV)
The nudity of Isaiah was a sign that not only would the Assyrians overcome Egypt and Ethiopia, they would make all prisoners (regardless of age) march into captivity completely naked (Isaiah 20:3 - 4)!
As you can see, the God of the Universe had a reason for telling Isaiah to do this. He was giving a sign and warning to the Egyptians and Ethiopians that would make them afraid to do the things they were doing. He was showing them in this prophetic action that they would become slaves of the Assyrians under Sargon's family.
Sargon II was an Assyrian king who began the offensive to destroy the lands of the south including Israel, Egypt and Ethiopia, which was the dream of his father Tilgath-pileser. Sargon took the name which means "God has appointed the king" but he died in battle or by homicide. Sargon's son, Sennacherib, continued his father's goal of destroying Egypt by bringing the empire's borders to the Nile Delta region.
Pharaoh Psamsik of the 26th Dynasty tried to reunite his country for a time but then the Babylonians, and later the Persians, invaded Assyria and took over their empire. The land became a Persian satrapy for a time when Cambyses conquered it. This all happened in the days following the fall of Samaria and the deportation of the Israelites to the land of the Medes and Assyrians.
The Kingdom of Judah were to be the next ones to be conquered and made slaves to the Babylonian king Nebuchadnezzar. The Assyrians today are the people who inhabit eastern Europe and Austria.
The captivity and enslavement of Israelites will happen again just before the return of Jesus Christ at his second coming and this is the message Isaiah was showing those who would listen. The Bible always has a former and latter fulfillment.
Egypt was one of the greatest empires in history. At the time of Isaiah the prophet, the 18th, 19th and probably the twentieth dynasties had already been on the throne and died. The empire, however, was already beginning to weaken as Libyan or Ethiopian pharaohs were on the throne but for them to think that they could ever be enslaved was a laughable thing.
Many were led away in abject slavery and were treated poorly by the rest of the inhabitants of the known world. Arabians captured them even as late as the 18th and 19th centuries to be sold as slaves to other nations. These slave ships were the most shameful behavior in all of human history.
God had Isaiah deliver his message naked and out of control, which is considered one of the most shameful things to happen to a man, so that the peoples of Egypt and Ethiopia would know what could (and would) happen to them.