Question

Why did many south vietnamese flee their country after north vietnam invaded saigon

Answer 1

They was working for US in the war, naturally they want to come with US army.

Answer 2

The South had its own insurgency, that developed through disgust with the corruption of the Southern government, only democratic by name. The North infiltrated and encouraged this movement, while mutilating and murdering any opposition they encountered. This was sometimes done in Vietnam, but too little and too late.