What major factors contributed to the demographic changes in the English colonies during the eighteenth century?

One major demographic change was that the colonies became more populated. Cheaper ocean travel along with better conditions in Britain led to a decline in indentured servitude—more people could afford the voyage. American colonial cities such as Boston and Philadelphia were rapidly growing places with a need for laborers. Immigrants from Ireland, Scotland, and Germany also came to the New World—this is in contrast with the mainly English immigration of the previous century. Many of these immigrants moved West in search of cheap land. The first British settlements west of the Appalachian Mountains date from this period. American colonists also had high birthrates; this made the American colonies "younger" than the British islands. This youthfulness would fuel the push to explore westward as more young people desired to have their own farmland. This youthfulness would also help to fuel an "American" identity as the young people who had never visited Britain had stronger ties to America than they did Britain.
Another demographic shift was an increase in African slavery. The sugar market of the Caribbean suffered a market decline and many sugar plantation owners moved to the Deep South, taking their slaves with them.

The English colonies of the Americas grew significantly during the 1700s. It has been noted that the population doubled almost every 25 years. There are a few factors that contributed to this growth in population.
In the 1700s, travel to the Americas from Europe increased when compared to travel in the 1600s. The price of travel also declined around this time. In the fifty year period between 1720 and 1770, the cost of transport across the Atlantic declined by about 50%. This meant that more people who were willing to head to the Americas were now able to afford the trip and act upon their desires. Many Europeans came to the Americas for greater freedom that was not afforded to them in their home countries.
Additionally, over 1 million slaves were brought to the Americas. While many of them ended up in the Caribbean, a significant amount also ended up in the thirteen colonies. They did not come of their own free will, but they still contributed to the growth of the English colonies.
http://nationalhumanitiescenter.org/pds/becomingamer/growth/text3/text3read.htm

The English colonies saw a major shift in its demographic during the eighteenth century. The fifty years spanning between 1700 and 1750 marked a dramatic increase in population size from 250,000 to one million individuals. By 1775, it is estimated that 2.5 million people were living in the colonies. On a more specific level, this looks like the following:
Between 1660 and 1760, there was an increase in Africans living in the colonies from 2,920 to 300,000. This number then jumped to 500,000 by 1775.
A shift from most immigrants arriving in the colonies being from England in the seventeenth century to being from Wales, Scotland, Northern Ireland, and Germany in the eighteenth century.
The reasons behind this shift are quite simple:
First, mass immigration from various parts of Europe rather than simply England created a greater, more diverse spectrum of colonists. Individuals were attracted by the economic promise of fertile American land and the opportunity to escape religious persecution back home. 
Second, the large number of Africans living in the colonies was singularly the result of the Transatlantic Slave Trade, during which Africans were captured, forced onto European ships, and sold as slaves to colonists, with ninety percent of this population living in bondage in the Southern colonies.