How can effects of one migration become a cause of another migration?

One of the main ways in which migration patterns can affect other migrations is that when one large group of people moves to another area, this limits the amount of available resources in the area, and often forces the people who were there first to seek new land--thus forcing those new people off their land as well, in something of a never-ending cycle. 

Where people were very broke no jobs were available and everyone was struggling