Check out this book: http://books.google.ca/books?id=5HCaqYbD5t0C&printsec=frontcover&dq=genghis+khan&hl=en&sa=X&ei=a13zU5bNA-OmigKj5oDQAw&ved=0CBoQ6AEwAQ#v=onepage&q=genghis%20khan&f=false

It provides a pretty good overview of the potential reasons/factors for the invasions, focusing obviously on the mongols. A very short summary is: Wide open plains for miles and miles makes defense of territory difficult (A side note to this, when the mongols invaded russia they controlled the southern plains, but had difficulty in, and never fully controlled, the forested, more easily defensible north). Diet helped too, high protein meat based diets made them generally healthier than their sedentary opponents who relied on grains. More mobile armies with the necessarry amount of animals to move rapidly, outpacing and outmaneuvering their opponents. Their political structure could also help, as loyalty and kinship ties were different in their societies. It was harder to get to the position of Genghis Khan, but once you were there, you kind of were there to stay until you died, unlike the settled nations were rivalries ran very deep between nobles and royalty.

That is a very brief summary of a very complicated topic. There are many more factors that I did not touch on.