it's two things.
time dwindling and the other team adding points.
the nowcast is rising because o's numbers are getting better.
you can see o's numbers getting better in the ev calculator, which is hardly time-related. ev calculator was 299 a few days ago. now it is 303.
i think sam wang conveys time changes better than nate silver in the chart at top left:
http://election.princeton.edu/electoral-college-map/