Which social forces were most responsible for the new imperialism that began in the late 19th and early 20th century?1850-1920 imperialism
I would say that the social force most responsible for this was ethnocentrism. This could be seen in the idea of Manifest Destiny and in the idea of the White Man's Burden that was so important in justifying this later imperialism that you are asking about.
White Americans tended to believe that they were morally and culturally better than the people they were setting out to conquer. They believed that they could help to civilize the people of places like the Philippines. This desire to civilize and Christianize (though most Filipinos, at least, were already Christian) various places was a very strong force that pushed the new imperialism along.