I would point out one additional factor. For a period of time after rebuilding/repair, the soil has not settled and is more prone to erosion. If the flood wall was overtopped and water undermined the newly compacted soil, that could have caused subsidence and collapse.
Also, there are persistent stories that a drifting barge or boat struck the flood wall and breached it. Certainly there was lots of stuff floating around -- photos of the 17th St. Canal showed a backup of floating debris at every bridge.
Yes, I have read about the possibility of non-compaction being a potential source, but this of course means that the upgrade allowed a potentially devistating risk for several years following the upgrade. I don't really know if an engineer would do work this way. It sounds like negligence to me. It would be like building a highway overpass and letting cars use it before the cement was cured to full strength. I hope this is just a rumour and not the reality of construction in N.O. But we will all get a chance to see the report, and see what it says.