If you look at the lower-panel cross-section at one end, you might notice the tensioning cables are in the upper part of the slab, not the lower. This suggests the slab was designed to resist bending from upward forces on the slab. This is consistent with the support that cable stays from a vertical pylon would provide. Since there was apparently nothing acting on the slab in an upward direction, such as would be the case with cable stays from a pylon after construction completion, or temporary falsework or supports during construction, all the tension in the slab tendons were acting to bend the structure downward in addition to the dead load of the structure. I think any tampering with the tension of the tendons that morning (as reported) could have contributed to starting the collapse. I think this structure had a critical erection schedule that the contractor failed to follow.