Why did the plane ticket cost go DOWN?

It is very possible that the airlines saw there was a high demand for the rout you are flying and added a flight, but tickets prices are influenced by all kinds things. They have mathematical equations run through computers that they use to set there ticket prices. In your case a flight could have been added, there was a large number of cancellations, or a large group canceled, the airline might not have released all tickets at once but in waves. If your looking for the cheapest ticket, sometimes if you wait till the last minute prices will drop because airlines want to fill the flight.