Caveat: I obviously don't know the details and I'm also not in any physical engineering or construction field.

This building was also literally right on the beach (map screenshot) that was on a teeny tiny little isthmus between the atlantic ocean and biscayne bay (second map screenshot). Given that it's florida, the bedrock is most likely just a pile of sand and shells or at best like limestone.

Edit: Roof work was apparently being done on the building at the time, per the mayor (who "doesn't believe it was the reason for the collapse") but it could've been a factor

Couple structural collapse episodes of Well There's Your Problem for the more general answers to "how things like this happen" (the short version is capitalism):

• Hyatt Regency Walkway Collapse, ft. Kate Wagner of McMansion Hell
• Sampoong Department Store Collapse only their second ep so they're still finding their footing a little
• Florida International University footbridge collapse