A dropped container is almost always a twist lock that was not fully engaged.
After landing the box, check that all four locks have rotated fully and the handle sits in its detent. Half-engaged locks look closed from the cab.
Inspect the lock body for cracks at the weld collar — that is where fatigue starts on a chassis that has run loaded for years.
Replace locks in sets of four. A new lock next to three worn ones takes all the load and fails first.