Well, according to an article on Genealogy.com, middle names were hardly used in the United States until the nineteenth century. In fact, although my grandparents were born closer to the beginning of the twentieth century, only one of them has a middle name. So, one middle name in general has not been used forever.
In my opinion, two middle names may eventually become the new thing. Although this is not mentioned in the article, it seems logical that some people may have started choosing middle names to separate one John Smith from the other. Now, with even more people in the world, perhaps some people are choosing two middle names so that Elizabeth Rose Victoria Jones can be separate from Elizabeth Rose Amelia Jones. In other words, now their are so many popular first and middle name combinations that a third name has to be added to distinguish individuals further.
My other theory is that with access to naming websites, people have found too many names that they like so they just add more middle names because they can't decide which ones to get rid of.
BQ: Miriam Eliza Anne Scarlett (Miriam Anne being my current name.)