The storm surge posed a different stress on the levees of New Orleans. A rapidly-rising water level, and water pressure coming from directions that the levees weren't designed to take.
A composite levee (base and fill are rocks/gravel/sand-earth) must be designed against water pressures from a certain direction. When the water pressures come from a different direction, the levee gets stresses it wasn't designed for.
'Nother levee question: when were they built? Some must date back to the Hughey Long days. Long, if you remember your history, was one of the most corrupt politicians to ever have power anywhere in the US. He was assassinated, as I recall. Wouldn't it be a strange irony indeed if the failed levees weren't properly built in the first place, because Long put some of the levee money in his pocket? It might be traceable.
If that scenario did take place, lo these decades ago, then Hughey Long is getting his final revenge on N'awlins.
Oh well, it's all up to FEMA now.