There are many factors that cause migration of people from villages to cities. Some villagers voluntarily move to the cities in search for jobs and better civic and health facilities, etc. Others are forced to migrate when natural disasters like flood, storm, drought, famine, etc. destroy their houses and properties. History has records of large scale migrations caused by wars. Also, many villagers who are better off than others manage to send their children to study in the cities.
In the lesson 'Lost Spring', Saheb and his family migrates to Seemapuri from Dhaka after their houses were destroyed in the storms.