Been there...
All too often a new or cheap person is hanging the ceiling and uses less wire supports than indicated. I gutted a former post office in college (The owner hired me to gut it). Some sections of drop ceiling were supported about every 4 ft, stagger fashion, and other sections were supported about every 10 ft. In some areas the supports were random-like too.
In one section I cut a couple wires and a couple hundred square ft of ceiling came down.
If the ceiling was anchored with roofing nails and tie wires then it frequently happens that the nail will work out over time. Vibrations from truck and air traffic is enough over a long enough time. There are screw anchors that work much better.
Phil