So the markets are reacting to news that a "compromise" has been reached on a "stimulus package". So maybe they were holding their breaths for the past couple days, wondering if the President and Congress would act like adults in the public interest.
Since they haven't the markets are doing the whole scampering chickens routine.
Mission Accomplished Again, George.
UPDATE: A neat feature that I had no idea existed when I linked to the little graph from Bloomberg is that it updated itself as the day went on. So, we now have the result of another wild day, rather than a snapshot of one moment early in the day.