There were some large blocks crossing in that last hour, and in that last half hour, there was a CUMULATIVE large amount of shares traded. That's why I started posting the volume every 10 minutes in the last half hour -- it was a large percentage of the daily volume.

Here are some volume snapshots:

Noon 54,263
3:30 153,263
3:40 187,263
3:50 256.234
4:00 384,480