The theory is always- Active site, activity caused the collapse. Probably not cheap steal or weak concrete. Seeing the rush to complete such buildings, I’ve always been concerned that loading the post tension concrete with pallets of building materials, sheet rock or glass before it’s cured is a bad idea. The temporary form supports give a false sense of security.
I'm not so sure about the concrete -- it appears to have separated very easily from the rebar, with only a few attached pieces. That should not happen absent some kind of force that would destroy the concrete other than a simple gravitationally induced collapse. And there is no evidence of any such other external force.