At least part of the answer is that God confused the language of the inhabitants of Babel. But that just raises the question how he did that, too. How he accomplished these ends, we are not specifically told and there is no way to know for sure. But we should bear in mind that God often accomplishes things through unwitting human agents. It seems to me that, if this was the first city and tower of the post-Flood world, and if they told themselves they would make a name for themselves and reach to heaven, they had grand ambitions, as Moses would have known the Babylonians did have. Different men might well jockey for position as leader; and one might well get the notion of using different words for things, as signs of allegiance and as a secret code indecipherable to enemies. Thus ambition would lead to speaking in competing codes; as a result, trust would evaporate and work would stop. The scattering would come when the people began to view each other as not just rivals, but as dangerous enemies. And that is just what we might expect of Noah, who was still on hand if all of living humanity were united, and his more decent offspring. They certainly would not trust the likes of Ham, Canaan, and especially Nimrod. Moreover, Noah might tell the people that God intended that they split up and go their separate ways. But, of course, this is all purely speculative. Again, we simply do not know.