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 the Assyrians would overcome Egypt and Ethiopia. It also symbolized that all the prisoners (regardless of age) would 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.
The Assyrians
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 that 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 was to be the next one to be conquered and made slaves to the Babylonian king Nebuchadnezzar. The Assyrians today are the people who inhabit eastern Europe and Austria.
Unbelievable
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. To them, the idea that they could ever be enslaved was laughable. They soon had a rude awakening, however, when many of them were led away in abject slavery and were treated poorly by the rest of the inhabitants of the known world.
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.