I think part of it depends on when your ancestors moved. It could be that new land was made available to settlers. Look at the history of the place where your ancestors moved from (was there a natural disaster?), as well as the place where they moved to (was there a gold rush?) for clues. Maybe some of your ancestors' relatives had already moved there and told your ancestors how wonderful Alabama was. Sometimes entire church groups moved.
My ancestors moved across the border from Harris Co., GA to Russell Co., AL. I am not sure what they gained by making that move. Of course after that they moved to Columbia Co., Fl.