Airlines raise, and lower, prices constantly. When prices are increased they get immediate feedback on how the new price is affecting booking. If they see too much of a drop in bookings then the price increase fails.

There are two ways that an airline can raise prices. First, they can increase the actual fares and they do. Another way is to decrease the number of tickets that they make available at the lower fares. If they were selling up to 20 of the cheapest fares on a flight they can reduce that to 15. The result is that they've raised the price on five of the seats.