Individuals migrate because they think that they can improve their own lives or those of their families by doing so. Economic migration is triggered by the knowledge (or belief) that better economic opportunities exist in some other place.
People migrate for many reasons. These include climate, natural disasters, personal beliefs, etc.
If natural disasters were to cause major damage to civilizations, citizens might have to do a permanent migration, this is where they find a new place to live. Or when it gets too cold they might do a periodic migration, this is whereas in the winter they move to a warmer location, just like bird migrations. Or they might migrate to a new area if they believe it is better and can improve their lives.
They wanted a new strong national government with checks and balances. they didn't want the 3 branches to abuse their power with the state so they wanted them to check each other
