Marshall - 9:33 pm on Aug 16, 2012 (gmt 0)

IMHO I believe it stems from sites trying to get more information above the fold by expanding the content section of the site and using dynamic drop down style menus either above or below the header. It also makes it easier to do a responsive design if you are not trying to relocate the navigation. However, I don't think left hand navigation is dead otherwise and is still used on the majority of sites.

Marshall