The reasons why groups of people leave a particular place are known as

The reasons why groups of people leave a "particular place" are known as 'pull factors'.

Explanation:

As the name suggests, there are certain reasons that firstly attract, and then fetch people out of their places in order to move to some other place that is more promising with regard to better earning opportunities, better standard of living, better healthcare facilities, better education facilities, better governance, more liberty, etc. These reasons are thus called 'pull factors'.