It's easy to second guess what happened and why after the fact. When the place was built I'm sure there were soils test & such done. The engineers know what kind of terrain they are building on. That cavern could have been 1000 feet below and a mile away at the time, unknown and undetectable. Underground water flow, caving and sluffing of the top and sides make those things move through the earth. It could have opened up anywhere.
Perhaps, this is conjecture, vibrations from the building through the earth could have caused the cavern roof to sluff beneath it, but I would think if that were the case, it would more likely have opened under the parking lot from vehicular vibration, or under the highway nearby. Or stormwater drainage, changed from the even absorbtion of the open fields that used to be there to concentrated points by building streets, buildings, etc. could have been a contributing factor in destroying the integrity of the ground below.
As far as slab reinforcement, that is not necessarily required. The structural foundations would have reinforcement, but a floor slab, especially if floating, not necessarily. It's just an interior floor slab with a very light load rating. No machinery, no concentrated loads. Reinforcement would be at the discretion of the engineers. Sometimes you don't want reinforcement, you want individual sections of a slab to float independantly and not transfer their stresses to other parts of the structure.
Yeah, a real bummer---but at least nobody was hurt, apparently (so far) no infrastructure or utility damage.
Sometimes shit just happens.