Somewhere towards the mountains, something subterranean tunneled after some prey. In the process of the hunt, it collapses an underground waterway. Water then found a new way.
In the city, the two large eastern quarters changed. The soil became damp for reasons beyond divination. Larger buildings began to slide slowly into the earth. It is a relentless, decades long process. People simply adapted.
In the beginning, the wetness was only a slight dampness. However that dampness went to the bones of the Earth. Most buildings sank a handspan or so. A few buildings toppled, most slid into the soft ground fairly evenly. New wooden boardwalk sidewalks butted up to newly raised doorways.
With the moisture came other problems. Regular scrapings kept the mold at bay. Walls were thickly plastered to keep out the moisture. The bigger the building the farther it sank. Handspands became cubits. Soon doors were added to the second stories, with stairways leading to the boardwalks. First story windows and doors were bricked over, to keep the mud from sloshing inside. Every building in the district now has a basement that was once the first/ ground floor. Third and fourth stories were added. Bridges were built connecting family buildings.
In some places there are second tier walkways, some two stories above the muck level. After two generations of sliding, some enterprising people build tile lined tunnels to cross the streets at the original ground level.
The sinking has created a new profession… Muckmen. The Muckmen wear small barrels on their feet, allowing them to move across the muck without sinking. Muckmen are messengers, builders, tunnelers, and any other job involving moving things across the muck.