Very complex question.
First, poor government oversight by the Army Corp of Engineers during the construction process, which allowed use of sub-standard soils (too low in clay content).
Second, poor inspection and testing post-construction.
Third, slow response for the teams trying to establish proper drainage. Caused the water to flow through the canals too fast, which created too much drag (specifically, a type of friction between a solid and a fluid). The drag put too much stress on the levees. They failed by undercutting at the weakest points.