Ceramic often have powerful stresses trapped within the material. An area of weakness such as a small crack or chip can allow these to be released and a small dimensional change, e.g. from heat can be the final straw.
Lighting can often be the culprit. Had the cup recently been moved? Had the lighting been changed? What is the difference between daytime and nighttime temperatures in that part of the museum? Was there any vibration from machinery nearby (e.g. workmen drilling in a nearby wall).
Until something like this happens, its easy to forget or ignore how much stress an object is subjected to – for example from poor mounting or handling or, as in this case, its own construction.
You may also find that repairing the saucer is not straightforward – when the ceramic breaks, the internal stresses may result in permanent deformation so the two parts will not join perfectly. If making the repair, please use a reversible adhesive such as Acryloid B72 (Paraloid B72 in the UK) in acetone, which is not stronger than the body of the ceramic.