In order to talk about why cultures change, we first have to have some idea as to what culture is. Let us look at the definition of culture from the link below. It says that
In social science, culture is all that in human society which is socially rather than biologically transmitted...
This means that the term "culture" covers pretty much everything about our society -- what religion, if any, people follow; what music they like; how they are educated.
When we look at things in this way, cultural change can occur for any number of reasons. The foods that we eat in a culture can change when immigrants come to the country and bring their cuisine with them. The music we listen to can change in that same way. Education can change when there are preceived threats to our country -- this happened when science and math education boomed after the Russians launched Sputnik.
Since culture is such a broad thing, many things can change culture. However, I would argue that the most likely cause of cultural change is contact with other cultures whether that be through immigration, through media (seeing other cultures on TV, etc) or other types of contact.