There is some method in the madness: essentially the airline distributes the available seats over the various fare categories and passengers can pick their ticket on a first come basis. Many airlines allow travel agents to "hold" tickets for 24 hours and you could have seen some tickets released when people cancelled their travel plans. It also could have been the airline looking at the ticket sales, deciding to throw in a new batch of low cost tickets.
If you have to travel and you see a nice fare: make the reservation now, you run the risk of paying a lot more if you wait. You can allow to wait for a better price if you can entirely cancel your trip.