Genesis 11

 After the Lord had commanded Noah and his posterity to settle though out the land, many disobeyed and grouped together as - 

"...they found a plain in the land of Shinar; and they dwelt there."  (Genesis 11:2)

and planned - 

"...let us make brick, and burn them throughly. And they had brick for stone, and slime had they for mortar.

4 And they said, Go to, let us build us a city and a tower, whose top may reach unto heaven; and let us make us a name, lest we be scattered abroad upon the face of the whole earth."  (Genesis 11:3–4)

Josephus claims that one motive was so the Lord couldn't flood them again, obviously unreasonable.

Thus - 

"...the Lord said, Behold, the people is one, and they have all one language; and this they begin to do: and now nothing will be restrained from them, which they have imagined to do.

7 Go to, let us go down, and there confound their language, that they may not understand one another’s speech.

8 So the Lord scattered them abroad from thence upon the face of all the earth: and they left off to build the city.

9 Therefore is the name of it called Babel; because the Lord did there confound the language of all the earth: and from thence did the Lord scatter them abroad upon the face of all the earth."  (Genesis 11:6–9)