This depended on three major factors:
1. The weather- If a certain group was not able to survive in temperatures/soil conditions, they would move.
2. The surrounding people- If an aggressive group was moving nearby a peaceful group, the peaceful group may choose to leave in order to stay alive.
3. The animals- hunters and gatherers hunted animals and gathered vegetation to stay alive. Say winter killed the vegetation and the animals migrated, the hunter-gatherers would follow their food, leaving their region.