Is there some historical fact that might have led to such a radical change based on the message of one Jew?
Why, yes. Yes, there is.
I think we often underestimate just how familiar Israel's neighbors were with the notorious God of Israel. Through various ways, Israel's neighbors knew generally who God was, what He has done, who His people were, and were capable of placing unfolding events related to Isreal and her God into their proper contexts.
Here are a couple of examples:
Nehemiah 6:16 And when all our enemies heard of it, all the nations around us were afraid and fell greatly in their own esteem, for they perceived that this work had been accomplished with the help of our God.
2 Chronicles 20:29-30 29 And the fear of God came on all the kingdoms of the countries when they heard that the Lord had fought against the enemies of Israel. 30 So the realm of Jehoshaphat was quiet, for his God gave him rest all around.
Joshua 5:1 1 As soon as all the kings of the Amorites who were beyond the Jordan to the west, and all the kings of the Canaanites who were by the sea, heard that the Lord had dried up the waters of the Jordan for the people of Israel until they had crossed over, their hearts melted and there was no longer any spirit in them because of the people of Israel.
One of my favorite examples is of Rahab explaining why she was helping the Jewish spies. Obviously, she could tell these were Israelites, God's people, and so figured she should probably be on their good side.
Joshua 2:8-11 8 Before the men lay down, she came up to them on the roof 9 and said to the men, “I know that the Lord has given you the land, and that the fear of you has fallen upon us, and that all the inhabitants of the land melt away before you. 10 For we have heard how the Lord dried up the water of the Red Sea before you when you came out of Egypt, and what you did to the two kings of the Amorites who were beyond the Jordan, to Sihon and Og, whom you devoted to destruction. 11 And as soon as we heard it, our hearts melted, and there was no spirit left in any man because of you, for the Lord your God, he is God in the heavens above and on the earth beneath.
My other favorite example is of the Philistines. Here they are obviously aware of the significance of the Ark and are appropriately afraid, but this doesn't prevent them from fighting the Israelites.
1 Samuel 4:6-9 6 And when the Philistines heard the noise of the shouting, they said, “What does this great shouting in the camp of the Hebrews mean?” And when they learned that the ark of the Lord had come to the camp, 7 the Philistines were afraid, for they said, “A god has come into the camp.” And they said, “Woe to us! For nothing like this has happened before. 8 Woe to us! Who can deliver us from the power of these mighty gods? These are the gods who struck the Egyptians with every sort of plague in the wilderness. 9 Take courage, and be men, O Philistines, lest you become slaves to the Hebrews as they have been to you; be men and fight.”
So it seems to me that it is most likely that the Assyrians were similarly very familiar with the nation of Israel, her God, and the dreadful things he's done to other nations. All it took was one man who likely very much looked like an Israelite and talked like Israelite with the audacity to walk alone into the middle of their city and boldly proclaim, “Yet forty days, and Nineveh shall be overthrown!” for all of Nineveh to come to their knees in repentance before the fearsome God of Israel.