While the market started lower today it rallied throughout the day. While the market sold-off on high volume at the end, this shouldn't be too surprising. The market made a strong advance today and my guess is traders were taking profits off the table.
Here's the 5-day, 5-minute chart. Notice the uptrend we started mid-last week is still firmly intact.
Here's the 3-month daily chart. Notice we are starting to move away from the cluster of activity around the 200 day SMA and are going into rally mode.