Why did fighter jets just fly over?

The most visible reason is a planned flypast. In the United States the Air Force and Navy schedule flyovers for the national anthem at major games, for the funerals of veterans and for holidays such as Memorial Day and the Fourth of July; in Europe, national days and state funerals bring formations of Rafales, Typhoons or F-16s over the capital. These flights are approved months ahead, flown in tight formation at a few hundred feet and timed to the second, and the local base or the organisers usually announce them. A missing-man formation, in which one aircraft pulls up and away, marks a memorial.

Far more common, and rarely announced, are routine sorties. Fighters transit between bases, practise low-level navigation on published routes or return home in pairs to an airfield near a city. Military aircraft normally keep at least 1,000 ft (300 m) above built-up areas and stay subsonic outside designated zones, but a jet at that height still makes a startling noise, and afterburner or a hard turn amplifies it. Around active bases, night flying and weekend surges are part of the training calendar.

The third possibility is an alert launch. Air-defence fighters on quick-reaction alert take off within minutes to identify an aircraft that has stopped answering the radio, has no transponder or has entered restricted airspace, for instance around a presidential movement. They may fly low and fast and can be cleared to go supersonic, which is why some intercepts are followed by a sonic boom. Flight-tracking sites often show the tanker or the intercepted aircraft even when the fighters are hidden, and the base's public affairs office or NORAD will normally explain within hours.