What lead to the shutdown of several Spanish airports?

[8] The Spanish government has warned it will allow the military to take control of the country’s airspace unless air traffic controllers return to work. Officials said many had called in sick as part of a long-running pay dispute, causing the shutdown of several Spanish airports.
    Many passengers were left stranded inside planes on runways after the protest began this evening. It’s the start of a national holiday here, and the airports are crowded with people who’d been planning a long weekend away. All flights in and out of Madrid, Majorca, Menorca and Ibiza have been cancelled, [7] the result of a dispute over air traffic controllers’ hours and conditions. The airport authority Aena has called the action irresponsible, accusing the controllers of holding the country hostage.