My first guess is that a big sinkhole is the culprit.
Practically the whole city is built on a swamp filled in with sand and whatever. It could have been undermined.
Yes, but sinkholes happen further north.........very very few in southern florida.
Now being built on a swamp or a spring --maybe.
This said first witness said that that collapse started in the garage.
Turns out it's built on a barrier island. Barrier islands constantly move and change shape. They are basically giant sandbars. Not really the most stable place to build a high rise, but the beaches make them very attractive.
Turns out that there has been a couple of lawsuits by condo owners over cracked outside walls too. In 2001 and 2015.
And there had also been some recent construction work on the building. A construction crew that doesn't know what they are doing can cause catastrophic damage.