I gotta wonder, what sort of bedrock was it built on? If it's limestone like they have further north in FL, water movement underground could've undermined part of the foundation over the past 40ish years or so. The center section that collapsed first appears to be part of a taller building mass, where the elevator core was located. That part went first, and then the eastern/beachside mass collapsed inward on top of it. My guess is undermined foundation, whether it's from a sinkhole or underground stream (not knowing Miami very well...could it even be a busted stormwater pipe that contributed?).