Ticket prices and availability change in unpredictable manner. The general rule says : the sooner the departure date, the quicker the changes. Some people wait till the literal last minute to book their flight – when only a few seat left. Those seats “disappear” (are being booked) in a matter of minutes.
Tip: Always book your ticket at least a few weeks in advance. It also guarantees you the best price, as the airline still have a good number of seats on offer.