I don't know. I forgot to ask him. You might find some hints here:
http://www.bing.com/search?q=Why+did+Pablo+Picasso+invent+Cubism%3F&FORM=EDGNNC
Why did Pablo Picasso invent Cubism?
2 answers
I believe he "invented" it because he could draw things realistically very easily, and wanted a change.