Saheb's family migrated to India from Dhaka, Bangladesh, because of:

• Destruction of their homes and fields:

  Typhoons destroyed their homes and fields in Dhaka, leaving them without a livelihood.

• Search for a better life:

  They migrated in search of a better life and means of sustenance.

This is explained in the book "Lost Spring" by Anees Jung.

Source: India.com