I went on the Miami subreddit to search for local theories about why it happened so that you don't have to. Here are a few, including some that pass the buck around.

• In terms of buck passing, apparently they have a neighbor that was under construction and the people that lived in the now collapsed building complained about vibrations.
• Another theory is that recently someone saw cranes placing heavy machinery on the roof of the collapsed building
• BTW, since this building was built 40 years ago, it doesn't fall under building code made after hurricane Andrew, which the redditors are implying was a big turning point nearly 30 years ago. I'm not in construction so I wouldn't know whether or not the changes were drastic.
• Someone else is saying inspectors knew there was too much sand mixed into the concrete (due to lax building codes) but looked away because luxury condos pay a ton in taxes.
• And finally, of course it could just be a sinkhole. This is Florida after all, sinkholes are very common when you build on a fucking SWAMP right next to the fucking OCEAN.

So TLDR; capitalist greed is the most probable cause.

EDIT: Here's a firefighter photo of the scene. Here's surveillance footage of the actual collapse.