This is a good question. I am assuming that you are talking about the expansion of Americans within the continent of North America. When more settlers and immigrants came to America during the industrial revolution, many people moved to various part of the United States. There was a greater push to go South and West. Some of the reasons were on account of geography.
For one thing, people are always in search of new lands. As a place becomes more and more populated, there is less land for people, which means less land to farm. So, for geographic gain people began to move to other places within North America. People also moved due to the environment. Warmer temperatures means better harvests for certain types of food. Finally, there was also a great move to the West for gold.