One possible event with a lot of flexibility is to have a flood that happens periodically. Sea levels and river levels rise until they cover all land, or maybe just some land, so that people are forced to flee to higher ground or load themselves into boats and wait out the disaster. After some time, water levels finally recede and people can come back down and resume their lives. Obviously, no preparation would mean certain death, and much of society would have to be built to anticipate this flooding. Castles on high ground might be valued especially.
When the flood recedes, people need a system in place to restore order to society. Goods have to be protected so that they are not stolen by bandits post-flood before the owners get back to them. People who floated away need to find a way to return home.
A quasi-scientific justification isn't hard. You can reduce the phenomenon to tides. Fix an astronomical object, let's call it a blue moon, in a large elliptical orbit. Every X years, this elliptic trajectory aligns with Earth orbit so that the blue moon is very close to Earth and takes up a huge part of the sky. This blue moon exerts an extremely strong gravitational attraction to water for whatever reason; maybe it only attracts water. During the "Time of the Blue Moon", water levels rise to a crazy level and remain that way until the blue moon is gone. The blue moon could easily fit into societal mythology, as a cosmic force of purity, cleansing, etc.
Example: have a flood every 25 years, lasting 6 months. Everybody not fleeing to the mountains will need to find their own boat and stock it with supplies to wait out the flood. Societies will have to store food in advance to prepare. Some societies might even be built around barges designed to float so that they can continue to live and conduct business in Venice-like even during the flood. Military planners will have to take into account the possibility of enemies using the flood to slip past their fortresses and defenses.