ANSWER: We can find our answer regarding the naked preaching of Isaiah in the book named after him. He wrote most of his books between 757 to 696 B.C.
3 When Ashdod was captured (by the Assyrians under Sargon), the Lord said, 'My servant Isaiah (the prophet) has been going around naked and barefoot for three years. This is a sign of what will happen to Egypt and Ethiopia. (Isaiah 20:3)
Isaiah's nakedness 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 (verse 4)!
As you can see, the God of the Universe had a reason for telling him 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.
Pharaoh Psamsik of the 26th Dynasty tried to reunite his country for a time but then the Babylonians and later 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 land of Judah in the south of the holy land 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. This captivity and enslavement 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 the prophet the 18th and 19th and probably the twentieth dynasties had already been on the throne and died and this nation 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.