In the days before nation states, polities in South-East Asia were largely trading ports. These dots constituted the maritime routes along which fortune-seekers of old travelled.
All sorts – those with brains as much as those with brawn – went where conditions were most promising for the moment. These could be people who originated in the region or from outside the region.
This is my first point. Migration was the norm in South-East Asia. From founders of city ports like Parameswara down to the common Minangkabau, migration was a necessity.