While not quite a case of "the sky is falling!" many cockroaches like to hang out on ceilings in dark rooms and they can drop and flee in a second. Why they want to be out in a wide-open space, and upside-down, is a mystery, but if it's dark, they may be overhead.
Thankfully, most will scurry off into a less-conspicuous place once the lights come on or people approach. But unfortunately, if you're trying to catch a ceiling crawler it's difficult to knock them down without having them fall on top of you or to hit the ground running and disappear into an unreachable hiding place.
If you've seen cockroaches on the ceiling, it's likely they have cracks or areas of detailing such as crown molding to emerge from and return to in the daylight or well-lit hours.