It's really quite simple. Wall Street and the markets are news driven.
The media has done its dead-level best to hype the panic in order to create fear. The people respond,like the sheep that they are, and the herd-mentality takes over, and you have a run on toilet paper and people are fighting over the last bottle of water. The market reacts in kind.
The administration takes positive steps, reassures the public, and Wall Street rallies.