More than the limestone underfoot being eaten away?
Now granted, some of the city is sinking. But those are areas like South Beach which are not actually built on limestone at all but on sand. And as anybody should know, sand tends to sink. As a material to build on top of, it is barely more stable than silt.
And that kind of geology is nothing unique to there. Manhattan has the same issue, which is why you only have skyscrapers where the bedrock is near the surface. And the rest is all sitting on soil and fill, where with the right conditions it might also start to sink.
Here is a rather interesting article I found.
In other words, they are literally building on what are barrier islands and are shocked the land is sinking. Well, yes. Because barrier islands are as stable as a sandbar in the Mississippi, or the barrier islands off of Virginia and North Carolina.
And that will happen every time the foundation of your buildings is sand.